[V2] Add RevRec Performance Obligations Feature

[Smolations]

1. Refactors Settings and Client to allow them to be more configurable in a testing environment.
2. Creates the PerformanceObligation and PerformanceObligationList entities for the V2 client, core entities concerning the new (and upcoming) RevRec API features.

Examples

These Performance Obligations (i.e. POBs) cannot be created/edited from the client/API. However, there are avenues for this functionality (contact Recurly Customer Service for more information).

// getting:
var pob = PerformanceObligations.Get(id);

// listing:
var pobs = PerformanceObligations.List();

[8eth]

DevQA +1