Add factory function for meta class

In the same manner as the form class there now is a factory function for the
forms metaclass. Additionally, some documentation was added to affected
classes and functions.
Fix and extend some documentation.

This is used to document all features that are already documented with
docstrings. Additionally, some links were fixed and the intersphinx library was
extended by some external libraries that cross-reference documentation.
Add tests for new meta factory and fix a bug

- Fix a bug where the new base classes __init__ would not be inherited
- Introduce new fixtures that test all or only custom metaclasses
- Apply new fixtures to some existing tests and also to a new test
Fix PR comments on #63
Fix PEP8 and update py.test req.

PEP8 had some violations on our new stuff
py.test > 2.3 needed to support pytest functional fixtures
Merge pull request #61 from Javex/master

Add model metaclass factory and fix/extend some documentation