Updates for Metadata 2.1 (PEP 566)

Updated package_data documentation

enable pip cache in appveyor build

Fix documentation inconsistency on version specifier

The `,` is the equivalent of an "and".
Also, the pkg_resources section documents and behave like this too:

```
>>> pkg_resources.Requirement('requests>1,>2').specifier.contains('1.2.3')
False
>>> pkg_resources.Requirement('requests>1,>2').specifier.contains('2.3.4')
True
```

Revert "Prefer new_build_ext for Cython builds as recommended by Cython project. Fixes #1270." Reopens #1270. Fixes #1271.

This reverts commit 038baa16.

Import Cython.Compiler.Main as recommended by Cython project to improve Cython detection. Fixes #1229.

Reflect find_packages behavior in doc