Fix #280: Add help icon to header with links to Github and docs.

Just an idea I had while looking for easy bugs to mess around with. Adds a help icon to the header that, when hovered over, shows a popup below the header with a small paragraph describing DXR and two links to the Github page and the docs.

![screen shot 2014-12-28 at 11 02 37 pm](https://cloud.githubusercontent.com/assets/193106/5566728/53585828-8ee6-11e4-97d1-8491a2d3fdeb.png)

It uses CSS for the hover and has some padding around the box to allow for inaccurate mousing around the box.

I wasn't able to test how this looks / works with the navigation box as I couldn't find any tests that had files with the navigation box, nor could I figure out how to get it to appear myself. It shouldn't get in the way but I'm not 100% sure about that.

Lemme know what you think!