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.

This PR builds on my previous solar position PR. No additional incompatibilities were introduced in this PR.