Add Term, Payment, and an attributes accessor

Hello! @jasondew and I added `Payment` and `Term` to the models and services. I hope our style is OK :smile: 

Also, we added an `attributes` accessor to the `BaseModel`. This simplifies spitting out a hash of the ROXML attributes (mostly for console output that is easier on the eyes). If you're not a fan of that, it's not a deal breaker.

Let us know what we can do to help get this merged! Thanks!