Make coffee watch message more informative

When running under watchmedo, coffee doesn't display any useful
information when it recompiles a changed file, so we make watchmedo echo
that information instead.
Don't delete generated files from xmodule-assets

xmodule-assets creates coffeescript files in the output directories. On
its next run, it used to delete the javascript files compiled from those
coffee files. Now it doesn't which should make coffee have to do less
work.

Fixes LMS-451
Write a more informative message when watching coffee files

Add more information about what type of change has taken place when a
watched coffee file changes.
Only write to xmodule asset files that have changed

Use md5 checksumming to verify that we only write out xmodule asset
files whose contents differ from what we are about to write. This
minimizes thrashing of the other watchers.

Fixes LMS-452