WIP: Feature/docs

PyLTI documentation

Also visible at: http://pylti.readthedocs.org/en/feature-docs/
I would be fine with opening mit_lti_flask_sample on github.com, especially now that it has a [readme (in develop)](https://github.mit.edu/mitxlti/mit_lti_flask_sample/blob/develop/README.rst).

@Ferdi ?