Breaking out session creation in client module to a method.  Added some initial tests for that method.  Include requests library as a dependency of installing the sdk.
Add tests for get_canvas_session, get_auth_token, and get_default_headers.  Update AUTH_TOKEN default config value to be None.
Merge pull request #4 from Harvard-University-iCommons/feature/get_canvas_session

Feature/get canvas session
Merge branch 'develop' into feature/section_methods

Conflicts:
	canvas_sdk/client.py
	canvas_sdk/config.py
	tests/test_client.py