Inject CSS styles

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