Usb api

@devoncarew 

Add support to use the new usb device api. It allows the user to select from a list of available `adb` devices. It also allows to add a new device. This opens a device picker. Once selected, the permission is retained forever until revoked.

Should we add a material design button for `+` => add device ?

![screen shot 2014-11-12 at 11 06 34 pm](https://cloud.githubusercontent.com/assets/2711803/5024530/99391f2a-6ac0-11e4-93a1-b516d696f82b.png)

Can you move the implementation of the chrome.usb.getDevices and chrome.usb.getUserSelectedDevices into a new, separate dart library? Something like mobile/usb.dart, or similar. We can then call them here w/ less knowledge of JS interop and how those methods work.