Add InvitationRevokerWorker.

InvitationRevokerWorker is scheduled when an invitation is sent. It will
revoke the invitation if it is able to, and send the attendee an email.
Merge pull request #11 from net-engine/revoke_invitation_worker

Add InvitationRevokerWorker.
Work in progress.. Begins messaging for confirm / decline links
Refactored email_links_controller
Finishes off email links (awaiting payments controller)
Begins Braintree integration
Begins specs and implementation of Braintree payments
More payment form specs
Satisfies remaining payment specs
Adds before_action to payments
Merge pull request #13 from net-engine/braintree

Braintree integration, awesome work on the specs!
Merge branch 'master' into improving_token_security