add user_id to invitations database via migration
add model associations
add UserMailer to invitations controller
add model validations to spec
Merge branch 'master' into mailer-invitations
remove house column from invitations
update tests to respond to the new invitation model
add inviting user's email to the invitiation email
Merge pull request #22 from purple-martins-2013/mailer-invitations

Mailer invitations