Send replies to exchange meeting requests

* When the user selects accept/decline/tentative in MessageView, we now send
  an email to the organizer, with an iCalendar attachment indicating the reply
* Added a unit test for the reply case, but more tests to be added to handle
  other circumstances

Change-Id: Iff799d88a92b6546735bf4965b22febf3a82b56f
6 files changed