Feature/use absolute urls for client calls

Include unit tests for RequestContext class. Revert back to using absolute instead of relative imports within SDK. Inside of client base, expand truth checking of auth_token to include more than just None.  Use absolute urls instead of relative urls when making client put, get, post, and delete calls, which means that existing section methods and tests must form an absolute url based on request context and relative paths for methods.
The way the doc links work is that the only (some) prior versions have links rather than each version having a link.  We're currently on 2.3.0 and the closest link is to 2.0.0.  I'm not really sure what to do besides make a note about what version the SDK is currently.  What would you suggest?