* Fix async Python functors invoking from multiple C++ threads (#1587)

Ensure GIL is held during functor destruction.

* Add async Python callbacks test that runs in separate Python thread

In some cases the user of pythonbuf needs to allocate the internal
buffer to a specific size e.g. for performance or to enable synchronous
writes to the buffer.
By changing `pythonbuf::d_buffer` to be dynamically allocated we can now
enable these use-cases while still providing the default behavior of
allocating a 1024 byte internal buffer (through a default parameter).

fix some uninitialized members in `value_and_holder` for
some of the constructurs.

Found with coverity in a downstream project.

I found that the numpy array tests already contained an empty-shaped
array test, but none with data in it.

Following PEP 3118, scalars have an empty shape and ndim 0. This
works already and is now also documented/covered by a test.

On some linuxes, /usr/include belongs to GCC and the standard
libraries that work with clang are in /usr/lib/clang/8.0.0 or
some variation thereof.

This results in errors such as:

```
/../lib64/gcc/x86_64-pc-linux-gnu/8.3.0/../../../../include/c++/8.3.0/bits/cxxabi_init_exception.h:38:10: fatal error: 'stddef.h' file not found

```

during extraction.

Recent versions of clang.cindex include [code][1] that converts
the internal byte strings to python str for you.

  [1]: https://github.com/llvm-mirror/clang/blob/master/bindings/python/clang/cindex.py#L72

Commit 2b045757 ("Improve documentation related to inheritance. (#1676)") left off
a ':' from a hyperlink, which breaks the Travis CI build.

fix missing member initialization in pytypes: read-only dict.

Found with coverity in a downstream project.

Alternative implementation for #1657: if
we know that `obj` is never a `nullptr` [1], we should
not `nullptr`-check it *after* already dereferencing it.

[1] https://github.com/pybind/pybind11/pull/1657#issuecomment-452090058

* Adds section to the reference.
* Adds section to advanced classes page describing how to use `get_overload`.

Fix an uninitialized member in `type_record`.

Found with coverity in a downstream project.

* #1208 Bugfix thread kill wihile running pybind11 module

* #1208 Bugfix missing space after catch

* Always capitalize Eigen

* Fix spelling

I think this particular method binding should not be done with `PYBIND11_OVERLOAD_PURE` but instead `PYBIND11_OVERLOAD`.

Bazel has a "strict" build model that requires all C++ header files be compilable on their own, and thus must explicitly #include all headers they require (even if de facto header inclusion order means they'd get them "for free"). This adds a couple of headers that are needed (but missing) by this model.

* doc: Add note about casting from `None` to `T*`

* doc: reword 'none-to-pointer' note message

* doc: mention opaque types in 'none-to-pointer' note message

* Added __contains__ to stl bindings for maps

* Added note regarding make install

* Fixing order of arguments in call to PyErr_GivenExceptionMatches in pybind11::error_already_set.matches

* Added tests on error_already_set::matches fix for exception base classes

Since the module is named FindPythonLibsNew, PythonLibsNew_FOUND should be set appropriately.

* Fix warning that not including a cmake source or build dir will be a fatal error (it is now on newest CMakes)
    * Fixes appveyor
* Travis uses CMake 3.9 for more than a year now
* Travis dropped sudo: false in December
* Dropping Sphinx 2
- clang7: Suppress self-assign warnings; fix missing virtual dtors
- pypy:
  - Keep old version (newer stuff breaks)
  - Pin packages to extra index for speed
- travis:
  - Make docker explicit; remove docker if not needed
  - Make commands more verbose (for debugging / repro)
  - Make Ubuntu dist explicit per job
- Fix Windows
- Add names to travis