Feature/session expiration

Use new SessionInactivityExpiration RequestContext subclass for SDK calls made in sectioning (by swapping out parent class for subclass in utils).  Add config setting to expire sessions after certain amount of inactivity to production settings file.
I think we want to have -0.5b1 at the end of this line, so that it exactly matches the version that's in install_requires in canvas_python_sdk.  So the line would be:

git+ssh://git@github.com/penzance/canvas_python_sdk.git@v0.5b1#egg=canvas-python-sdk-0.5b1