Testing salt2 model error

 - Changes have only been made to models.py in both Models and SALT2Source classes in order to add SALT2 Model errors
 - Diagonal components tested against SNANA v 33.c. Reasonable agreement. Disagreement due to the way F0/F_tot is evaluated in this SNANA version using galactic extinction. Will probably match better in later versions
Comparison:
![snana_33c_sncosmo_modelerr_diag](https://cloud.githubusercontent.com/assets/2281358/2930683/7b56668a-d799-11e3-96c3-f4d4a15f61b5.png)
Cosmetic Changes: 
 - Added couple of helper functions which make life easier while testing
 - Changed names of functions like bandflux_rcov to restbandflux_rcov(self, band, phase) in the SALT2Source class.