Convert the gem to take in json keys documented by the docusign API instead of the ruby underscore keys currently used about docusign_rest

[tcopeland]

Good thoughts. It's true that every time we need to support a new endpoint (and sometimes new parameters) we need to add new code and release a new gem.

More generally, currently this gem is a rather thin wrapper over the API. A client has to know a fair bit about what's expected to make a call - that is, you have to know what to put in the options hash. Seems like we could do a little more in terms of encapsulation. On the upside, the current design does allow a client app to easily monkeypatch in methods for wrapping additional endpoints as needed.