Custom user-agent string

This PR adds a custom user-agent string. The setting for this is added to a new preferences tab. The UA defaults to the original, which is still dynamically created.

![screenshot from 2013-12-27 17 35 43](https://f.cloud.github.com/assets/4979708/1815688/8315620e-6f20-11e3-9188-5a7ad92f8076.png)