added recipient to model

-This allows us to track recipients. Ffor example we may want to email
all recipients a few days prior to expiration.
-It also allows us to pre-populate form data with the recipient email
address and validate it.
-create_bulk_invitation still do not require a recipient.
-send_bulk_invitations includes a recipient for each invitation sent
-Including the recipient email address in the invitation url is optional
so the upgrade is backwards compatible with previously sent invitations
using this format.  This will be removed at a later date.
-includes south migrations
-session is now passed the invitation_key object instead of just the key
value allowing more end user flexibility.