Inject CSS styles

Looks like styles are reloading the whole browser. This should inject them without a full reload. I tested with 10 simultaneous CSS file changes, seems to work well.

Also now has the cool "Injecting files into all connected browsers" dialog:
![image](https://cloud.githubusercontent.com/assets/925735/3394233/72ae955a-fcda-11e3-94d7-22fc8840e8e5.png)