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.
Can we point to the specific version of the requests library that we use, just in case there are changes in a future version that we don't support?