in-memory storage for testing

while testing my application, I found that I did not want recurly making real API calls. I briefly flirted with VCR, but decided I wanted something simpler, and decided to stub away recurly's HTTP interactions and replace them with a thing which simply stores records in memory. 

this is the initial form of that. it's incomplete, as seen by the various TODO comments and NotImplementedErrors raised, but I think this is something other recurly users would probably find useful, so wanted to open a PR and see what y'all at recurly think of it.