Add a simple version of a "get_count" utility method that return the len...

...gth of a call to get_all_list_data.  Added corresponding tests.  Update OAuth2Bearer class to raise an exception if the token passed in is empty, not just None.  Added a new test for that case.  Import RequestContext in base canvas_sdk init file for easier access.
You may want to document that this method may perform multiple API calls, with a worst-case complexity of O(n). Since the majority of SDK methods make exactly one call, it's probably worth calling out the exceptions to that.