Added mail notification when a new message is posted in the forums.

Added mail notification when a new message is posted in the forums.
Only users who "watch" the board receive notifications. To watch a board, go to the board and click on the "Watch" link.

Notifications are sent by MessageObserver observer.
GLoc was modified to use the mail template without language suffix when translated template (with language suffix) doesn't exist.
Usefull when there's no hard coded text in the mail tempate.

git-svn-id: http://redmine.rubyforge.org/svn/trunk@531 e93f8b46-1217-0410-a6f0-8f06a7374b81