Closes #1048, closes #1052.

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PR #880 changed the implementation of keep_alive to avoid weak
references when the nurse is pybind11-registered, but the documentation
didn't get updated to match.

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Fixes #1046.

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There are two separate additions:

1. `py::hash(obj)` is equivalent to the Python `hash(obj)`.
2. `.def(hash(py::self))` registers the hash function defined by
   `std::hash<T>` as the Python hash function.

The warning is shown at module initialization time (on import, not
when the functions are called). It's only visible when compiled in
debug mode.

This is analogous to `py::init()` vs `__init__` + placement-new.
`py::pickle()` reuses most of the implementation details of `py::init()`.

The lookup of the `self` type and value pointer are moved out of
template code and into `dispatcher`. This brings down the binary
size of constructors back to the level of the old placement-new
approach. (It also avoids a second lookup for `init_instance`.)

With this implementation, mixing old- and new-style constructors
in the same overload set may result in some runtime overhead for
temporary allocations/deallocations, but this should be fine as
old style constructors are phased out.

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Creating an instance of of a pybind11-bound type caused a reference leak in the
associated Python type object, which could prevent these from being collected
upon interpreter shutdown. This commit fixes that issue for all types that are
defined in a scope (e.g. a module). Unscoped anonymous types (e.g. custom
iterator types) always retain a positive reference count to prevent their
collection.

For the case of `pip install --user`, the header include dirs must
also include `pybind11.get_include(True)`.

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The default `install_headers` from `distutils` flattens all the headers
into a single directory -- `detail` subdirectory was lost. This commit
fixes this by overriding the setup with a custom header installer.

Tests are added to Travis to make sure `setup.py sdist` and `pip install`
do not miss any headers and that the directory structure is preserved.

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Newer clang produces additional warnings.

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The `latest` build remains as is, but all others are modified to:

* Use regular Python instead of conda. `pip install` is much faster
  than conda, but scipy isn't available. Numpy is still tested.

* Compile in debug mode instead of release.

* Skip CMake build tests. For some reason, CMake configuration is very
  slow on AppVeyor and these tests are almost entirely CMake.

The changes reduce build time to about 1/3 of the original. The `latest` 
config still covers scipy, release mode and the CMake build tests, so 
the others don't need to.

The current PYBIND11_INTERNALS_ID depends on the version of the library
in order to isolate binary incompatible internals capsules. However,
this does not preclude conflicts between modules built from different
(binary incompatible) commits with the same version number.

For example, if one module was built with an early v2.2.dev and
submitted to PyPI, it could not be loaded alongside a v2.2.x release
module -- it would segfault because of incompatible internals with
the same ID.

This PR changes the ID to depend on PYBIND11_INTERNALS_VERSION which is
independent of the main library version. It's an integer which should be
incremented whenever a binary incompatible change is made to internals.

PYBIND11_INTERNALS_KIND is also introduced for a similar reason.

The same versioning scheme is also applied to `type_info` and the
`module_local` type attribute.

The "see above" comment being referenced in the code comments isn't
"above" anymore; copy the later factory init comment into the first
constructor block to fix it.