Added force launch params.

Pushing doesn't happen if there have been no changes to an app since last launch; this added param allows a launch to be forced.
We force a launch when push is called externally, but don't force it when pushing is the result of a detected file change.
Added comments explaining launch forcing.
Merge pull request #345 from MobileChromeApps/force-launch

Added the ability to force app launches when pushing.