Several improvements on invite module

Hi all,
I modified invite module in order to accomplish two main improvements:

- making module fully translatable (several lmessages are not translated)
- make email body text 'intelligent' and fully translatable. Now, it is possible to user several variables into the text, so that they are translated when email is sent.

Unfortunately, I could not create a patch file, so I include the full modifire source.

Matteo