Adds 'emailer_send_prepare' and 'emailer_send' hooks
Adds relative documentation in doc/Plugins
Adds optional parameter 'uid' to Emailer::send()
that is the user id of the recipient of the mail.
Make enotify pass it to Emailer::send()
Merge pull request #1266 from fabrixxm/feature-hook-emailer_send

Adds 'emailer_send_prepare' and 'emailer_send' hooks