Datetime tweaks to improve numpy 1.6/1.7 datetime64 compatibility
I've tried to isolate pandas usage of the datetime64 from the aspects of the numpy
API which would be nice to evolve. Pandas wasn't directly using this API for
functionality or interoperability, but more as a consequence of borrowing some
of the functional datetime operations from the numpy codebase.

As a result of doing this, some datetime64-related bugs were exposed in numpy
master, and I've submitted pull requests to fix them.
