Minor fixes to bt.py, and included pint.models.bt in setup.py

Three changes:

1) The phase calculation in BT_delay was C-style. This is more pythonic.

2) The difference in omega due to omdot was missing. Question: is there a neater way to do the conversion with astropy? There does not seem to be a SECS_PER_YEAR one for instance.

3) Included the BT model in the setup