Prompt user to send invitations if leaving page

If a user has added new invitations to the invitation list, prompt
confirmation so that user knows that he must finish a second step to
actually send the invitations.
readme
Bump version numbers to 1.3.3
Update POT for 1.3.3
Switch to URL arguments rather than URL chunks for accept-invitation and opt-out page

Changes in BuddyPress 2.1 (https://buddypress.trac.wordpress.org/changeset/8658)
broke IA's old system for catching 'accept-invitation' and 'opt-out' URLs. We
avoid this problem by switching to URL arguments.

Backward compatibility is included for existing invitations.

Fixes #119
Fix plus sign emails before pre-filling them into the registration form
Readme
Fix opt-out Accept Invitation button for +-sign emails
Get email from $_GET rather than action variables on opt-out page
Fix form submit for opt-out
More fixes for opt-out screen form submit
readme
Bump version numbers to 1.3.4
POT update for 1.3.4