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.
I suppose if someone is dealing with JSON data to begin with, then they'd want to send it over with the right content-type header.  The ability to specify a header with a method call and on the request_context should provide such users with that ability though.