COMPAT: Consider dropping numpy 1.6 support
## NumPy 1.6

NumPy 1.6 introduced numerous bugs around datetimes, forcing a plethora of hacks, and causing numerous stress-induced ulcers for an uncountably infinite number of people.

The time for pandas to lose this vestigial organ has come. The main reason for this is that datetime support is just broken, in nearly every sense of the word. Take a look at some of the conversion code in `pandas/core/common.py` and you'll see what I'm talking about.

I think we've probably reached a point where supporting numpy 1.6 is just going to cause more pain than it's worth. To me it makes _no_ sense to want to keep upgrading pandas while keeping `numpy <= 1.6.1`. If you want modern pandas, you need to use modern-ish versions of its dependencies.
