Adding an experimental remote control feature.
This allows pages using wtf.remote.connect() to slave themselves to a
controller that can clear or capture snapshots. This enables getting
snapshots from remote devices or browsers that cannot save files themselves
(such as Android/iOS/etc) and makes other browsers easier to work with
(CROS). Could use more features and such, but it seems to work.

Work on #313.
Merge pull request #316 from google/remote

Adding an experimental remote control feature.