PyPy support

This is a tentative to make peewee work on PyPy.

So far most of the issues I could find are related to the database packages that are compatible on PyPy/CPython.
This is still a WIP, I didn't make time to look at the MySQL support yet. I will try to have an update on that asap.

Please let me know if there are any questions so far, or there are better ways to handle the compatibility aspects.
Thanks in advance for reviewing this.