WIP: Feature/docs

PyLTI documentation

Also visible at: http://pylti.readthedocs.org/en/feature-docs/
Done.  I have linked 'LTI Spec' to the LTI spec on IMS, I have added a link to 'code.edx.org' for my mention of edX as an LTI Consumer and indicated that this code was written with edX in mind.