License compliance via an "about" menu item

Add an about menu item at the bottom of settings with the license for various dependencies.

Fixes #459

Slightly outdated screenshots (looks essentially the same, text has been modified a bit):

![screenshot_2014-10-21-22-56-29](https://cloud.githubusercontent.com/assets/512573/4730317/5da6fa16-5997-11e4-905f-500ec085652f.png)
![screenshot_2014-10-21-22-56-38](https://cloud.githubusercontent.com/assets/512573/4730319/63b11450-5997-11e4-9785-25b3cd455bb0.png)