FEATURE: remote scratchpad 

This prototype adds a new DevTools command ("firefoxos scratchpad")
to open a tweaked scratchpad window connected to the controlled 
b2g-desktop instance using a new custom "scratchpad actor", 
it supports 3 switchable contexts:

- current displayed FirefoxOS app context ("app")
- current FirefoxOS browser tab ("browser-tab")
- FirefoxOS Chrome shell.xul ("chrome")

Supported scratchpad features:
- run selection/buffer
- display selection/buffer
- load / save to file

Not supported scratchpad features:
- inspect (needs more work)
- reload and run
- reset variables

DEMO SCREENCAST: http://youtu.be/M8jW8F7OCJU