Backport MOODLE_27_STABLE to MOODLE_22_STABLE

* add back previous/next
* fix the file serving script to support the 2.2 version of send_stored_file()
fix grab redirect url
Update README
Port watcher/hd_user tables + file api support + some previous 2.X improvement - it is a massive mix of many commits from Jon Doane/David Zaharee/Jerome Mouneyrac
Downgrade the required version to the first 2.2
Fix various warnings and errors related to 2.7
Remove bad character
Bump version number for last upgrade block
Remove settype warning + empty variable warning
Check Moodle version for the fullname warnings by pass
Minimum Moodle version is the first 2.5
Fix empty user id for external user when sending email + create a function to send email to external user without warnings
Fix warning when no watchers on reply
Fix email_to_external_user to be 2.5 compatible