New module: screenshot

[sole]

Not sure which actor implements this (or if said actor is in simulators/phones!)-will add more info as I ask around.

[sole]

Alex told me:

for the screenshot, we are querying the device actor over here http://mxr.mozilla.org/mozilla-central/source/toolkit/devtools/server/actors/device.js#163

[wfwalker]

+1 would love to have this!

[fwenzel]

Would love to see this integrate with unit tests and such <3

[nicola]

++1