READY FOR REVIEW: Feature/docs

PyLTI documentation

Also visible at: http://pylti.readthedocs.org/en/feature-docs/
This comment wasn't fully addressed or answered (though we chatted a little about it in real life).  Please file new issues for setting a default request type and an issue to possibly replace the app piece of the decorator to a structure more explicit to what the decorator needs out of that huge object (i.e. the keys config).  Then reply to this with the issues to make cross referencing and context easier.