This patch brings the pure-python datetime more in-line with the C module. We have been running these modifications in PyPy2 stdlib for more than a year with no issue.

Includes:
- General PEP8/cleanups
- Better testing of argument types passed to constructors
- Removal of duplicate operations (in some paths values were checked twice for validity)
- Optimization of timedelta creation (brings it from 8-9usec to ~6 usec on CPython 3.3 on local machine)
- Enhancements/bug fixes in tests