Switch to standard coffee watcher

Using `ulimit -n` to set the limit much higher than the default of 256
in Darwin seems to avoid the `EMFILE` error that was plaguing our Mac
developers.
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
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
Make assets watchers run as singletons

Previously, multiple copies of the watchers started from the different
shells would run simultaneously, which left the possiblity of zombie
watchers, increased resource consumption, and incorrect results. This
fixes that problem by only starting a watcher if that same command isn't
already in the process list.

Fixes LMS-499
studio - removes course settings promo link and notice icons
Merge branch 'cale/singleton-watchers' into cale/stabilize-dev-env

Conflicts:
	CHANGELOG.rst
studio - revises styling of in-context notices UI
Merge branch 'cale/standard-coffee-watch' into cale/stabilize-dev-env

Conflicts:
	CHANGELOG.rst
	rakelib/assets.rake
Fix merge conflict in CHANGELOG.rst.
Merge pull request #174 from edx/cale/stabilize-dev-env

Cale/stabilize dev env
Fixed the preferences scope of xblock.

Added self to authors.

Conflicts:
	AUTHORS
	CHANGELOG.rst
pep8 fixes.
studio - unconditionalizes course summary info in settings
Merge pull request #222 from edx/felix/prefsscopefix3

Fixed the preferences scope of xblock.
Merge pull request #223 from edx/christina/pep8

pep8 fixes.
Fixed flakey show answer test
Fixed flakey navigation tests by changing css

Now only check for the css that appears when the accordion is done
Merge branch 'master' into christina/course-settings-drupal
Merge pull request #197 from edx/will/fix-video-acceptance-tests

Will/fix video acceptance tests