added mailer for when a student submits to an assignment
moved tests over to milestone submission model spec; the test for the URL is currently failing
cleanest version of milestone_submission_spec, still failing
Refactor tests and rename mailer.
Merge branch 'assignmentmail'

* assignmentmail:
  Refactor tests and rename mailer.
  cleanest version of milestone_submission_spec, still failing
  moved tests over to milestone submission model spec; the test for the URL is currently failing
  added mailer for when a student submits to an assignment