Notify employee or admin whenever there is a new leave-related activity

This is a really huge pull request. In essence, this pull request adds email notifications for the following events:

- Employee files a leave request (Email all HR of pending leave request)
- HR approves a leave request (Email employee about the approval)
- HR rejects a leave request (Email employee about the rejection)
- HR marks an approved leave request as taken (Email employee about the action)
- HR grants leave credits to an employee's leave account (Email employee about the action)

The email action is placed in a background process c/o [Resque](https://github.com/resque/resque).