Sending confirmation mail when user subscribe to event.

#4382 
![mail](https://f.cloud.github.com/assets/1222368/1482023/9520456c-46d6-11e3-95cd-8cbc4344e85d.png)
![event](https://f.cloud.github.com/assets/1222368/1482025/97bf2414-46d6-11e3-958e-b78228c821d6.png)