mailer: support "on behalf of" registration (Fix #837)

(contains 6 new language strings)
Fix #839: Trigger mailer plugin when removing user from attendee list (back+front)
version set to 2.0.3, index.html added to xml files as needed
removed wrong update server url from xml and db tables (#834)
Merge remote-tracking branch 'remotes/origin/development' into dev21

Conflicts:
	plugins/plg_finder_jem/jem.xml
alter code regarding "potential backward compatibility issues in Joomla 3"

things like
- db query -> execute()
- JRequest -> JInput
- JemModelAdmin to wrap prepareTable() conflict
and more

Note: Not fully tested, especially install/update untested yet!