Update toa.py

Corrects a bug when using barycentered data (observatory is not found, because "Barycenter" is not a key of `observatories`)
Update toa.py

Avoids raising ValueError if the observatory is Barycenter; corrects syntax error (using `is` instead of `==` for string comparison)
Create bt.py

This class provides an implementation of the BT model by Blandford & Teukolsky 1976, ApJ, 205, 580. Based on BTmodel.C in Tempo2. Tested to agree with the Tempo2 version to ~ 10ps
Merge pull request #10 from matteobachetti/patch-5

Update toa.py
Merge pull request #11 from matteobachetti/patch-6

Update toa.py
Merge pull request #12 from matteobachetti/patch-7

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