Refactor a bit modx_dispatch

Also, add a test for this behavior.

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

Highlights:
* The url '/shib-login/' interfaces with apache/mod_shib via
  request.META to handle shibboleth login and registrations
* Courses can designate 'enrollment_domains' to limit enrollment
  to users with a linked ExternalAuthMap verified by a particular
  identity provider
* Tests
* Logging

 Changes to be committed:

	new file:   common/djangoapps/external_auth/migrations/0001_initial.py
	new file:   common/djangoapps/external_auth/migrations/__init__.py
	new file:   common/djangoapps/external_auth/tests/test_shib.py
	modified:   common/djangoapps/external_auth/views.py
	modified:   common/djangoapps/student/views.py
	modified:   common/lib/xmodule/xmodule/course_module.py
	modified:   lms/djangoapps/courseware/access.py
	modified:   lms/djangoapps/courseware/tests/test_access.py
	modified:   lms/envs/common.py
	modified:   lms/envs/dev.py
	modified:   lms/envs/test.py
	modified:   lms/templates/courseware/course_about.html
	modified:   lms/templates/dashboard.html
	modified:   lms/templates/extauth_failure.html
	modified:   lms/templates/navigation.html
	modified:   lms/templates/register.html
	modified:   lms/templates/signup_modal.html
	modified:   lms/urls.py
	renamed:    lms/wsgi_apache.py -> lms/wsgi_apache_lms.py

When changing variable names for code review critique, accidental cut/paste prevented intended behavior.

Rename fields in CourseTaskLog, including a task_key column for indexing.  Use 'rescore' instead of 'regrade'.  Clean up task submission.

Add CourseTaskLog, and make calls to regrading.

Add regrading implementation, and hack the marshalling of request.