Add support for maintainer in PKG-INFO

Don't claim to provide the extra ''

Per PEP 345, metadata Version 1.2 should support the Author,
Maintainer, Author-Email and Maintainer-Email fields.

provide a failing test case for regression introduced in b2ea3c4a - spurious "Provides-Extra:" generated in metadata

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
```