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
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
This allows us to a) compile the coffeescript and sass from xmodules
using the new out-of-band method and b) reload xmodule static content
whenever it changes, which should make devs much happier.
