add course invitations
remove duplication
improve test coverage and ..
extend courses to have attending_invitation for easy access
remove left over controller
Merge pull request #17 from codebar/course-emails-and-other

Course emails and other
add session reminder email and ability to reject
invitation
add invitable concerns
add rake task for session reminders
add Reminders to make sure they only get sent out once
improve tests
utilise #attending_invitations
fix bogus spec
Merge pull request #18 from codebar/student-email-reminders

Student email reminders