TMY module

This PR consolidates the ```readtmy*``` modules into a ```tmy``` module. TMY dataframes are now tz_localized to TZ*3600 offsets.

```nosetests pvlib/test/test_tmy.py``` passes on python 2.7 and python 3.4.

I also changed the TMY3 date parser so that it builds each date time object instead of using ```pd.date_range(date[0], freq='1H', periods=len(date))```. The old method, while faster, would fail if there were any gaps in the data set. I don't have enough experience with the NREL TMY dataset to know if that is ever a problem, but it could easily be a problem if somebody wrote out their own TMY files. I avoided the 24 hour problem by subtracting an hour from the date set before sending it through the parser, and then adding a 1 hour relativetime.

This PR builds on my previous solar position PR. No additional incompatibilities were introduced in this PR.