Several Bug Fixes and Features for Invite Module

Tha attached patch does the following:

• Fixed bug where if the invitee signs in with a different email, they don't show up as 'joined' and are left as 'pending' -- now it changes the email to the invitees prefered address and marks them as joined.
• Can now set the From and Reply-To headers for invitations in settings
• Made both the template wording and user message in the email that's generated wrap at the standard 72 characters.
• Exported the help text to settings so you can customize what it sayson the invite page
• Can now set page title for the invite page in settings
• Added a unique id to the table of invited users on the invite page so that it can be better targeted by css.

I would be happy to ammend if necessary, or make further fixes/improvements