Fix the JSON encoding
get_schema no longer returns a voluptuous.Schema, it returns a dict so that the user can add more validator and then turn it into a voluptuous.Schema
make assignment_id and user_id not nullable for the Submit table
No longer require mimetype to be application/json, instead, try to load JSON if there is some there, else fail silently

Fix get_schema calls for the new semantics

Save the user ID when submitting a new homework for grading
Add wrapers for requests methods

Get rid of the forced headers

Add submit support