* Test dtype field order in numpy dtype tests

When running tests with NumPy 1.14 or later this test exposes the
"invalid buffer descriptor" error reported in #1274.

* Create dtype_ptr with ordered fields

* Fix casting of time points with non-system-clock duration on Windows

Add explicit `time_point_cast` to time point with duration of system
clock. Fixes Visual Studio compile error.

* Add test case for custom time points casting

In def_readonly and def_readwrite, there is an assertion that the member comes
from the class or a base class:

    static_assert(std::is_base_of<C, type>::value, "...");

However, if C and type are the same type, is_base_of will still only be true
if they are the same _non-union_ type.  This means we can't define accessors
for the members of a union type because of this assertion.

Update the assertion to test

    std::is_same<C, type>::value || std::is_base_of<C, type>::value

which will allow union types, or members of base classes.

Also add a basic unit test for accessing unions.

* 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`.