Added new realtime development task watch:ff.

Replaces the former gm + watch:gm tasks to aid in realtime development and debugging.

The new watch:ff task builds a new Firefox extension and autoloads it into Firefox in the most performant way possible.

Note that the stated performance goal is not accomplished yet; the current task averages at 2.8 secs (on Windows).
Clarified enforced exclusion of artifacts in watch files.
Merge pull request #133 from sun/grunt-watch-ff

Added new realtime development task watch:ff.