Fixed #908 -- Enable account notification for inviting friend to OH.

Resolves https://openhatch.org/bugs/issue908. Feel free to code review and make suggestions for improvement. 

New notification message after the Welcome to OH email has been sent to a friend (or enemy):

![openhatch-issue 908-invite-somone](https://f.cloud.github.com/assets/954858/1749991/1f1b9774-6560-11e3-8ac1-21f1ab567aa3.png)