Feature implementation: UI setting to stop trying to send messages after X hours/days/months.

A feature request implementation with [detailed documentation](https://docs.google.com/file/d/0BydWFsI3E-xERXVObXkyUkNEdk0/edit?usp=sharing). Soon a forum thread will be created on the [bitmessage forum](https://bitmessage.org/forum/index.php).