Added invitation model/controller

Not complete; mailer yet to come to send invites.
Invitation.sent_at not setting correctly now, may need mailer?
Probably need more tests overall.
Add invitation mailer
Add, update, configure gems
Add unique invitation tokens
Add realm status to sidebar
Replace guild progress with level & achievements
Remove signup link from navigation
Set sent_at on invitations
Add heroku gems
Update rails version, add .rvmrc