Updated .gitignore to exclude logs and :memory file
Started creating tests for AssignmentsController
Format date for assignment's deadline
Force DataMapper to raise an exception if a non-mocked DB call is made
Replaced 'create' method with 'new' on solution's spec (WiP)

This way, we avoid invoking an insert on the DB which raises an
exception as we're blocking all DB calls.
Currently there's 1 test failing because it involves some uniqueness
validations which require to be mocked, and the Assignment object isn't
getting correctly set its 'course' attribute