Features coming down the pipe will want to be able to:
* Refer to enrollments before they are actually activated (approval step).
* See what courses a user used to be enrolled in for when they re-enroll in
  the same course, or a different run of that course.
* Have different "modes" of enrolling in a course, representing things like
  honor certificate enrollment, auditing (no certs), etc.

This change adds an is_active flag and mode (with default being "honor").
The commit is only as large as it is because many parts of the codebase were
manipulating enrollments by adding and removing CourseEnrollment objects
directly. It was necessary to create classmethods on CourseEnrollment to
encapsulate this functionality and then port everything over to using them.

The migration to add columns has been tested on a prod replica, and seems to be
fine for running on a live system with single digit millions of rows of
enrollments.

Add audit log definition, and use for logging of logins in external_auth and student apps.  Move test_login to student app. Improve conditional tests for Shibboleth login logic.  (Does not include reconfiguring log settings.)

Clean up some pep8/pylint violations as well

which caused UnboundLocalError.  Added tests for this case

* Changed unicode test cases to ascii encoding
* Removed 'stanford' hardcoding in TOS logic in lieu of
  'SHIB_DISABLE_TOS' MIT_FEATURES flag
* made 'external_auth' always an installed_app in lms
* log.exception changd to log.error where appropriate

But: did not change skipping tests to changing settings, for
reasons stated here:
https://github.com/edx/edx-platform/pull/67#issuecomment-19790330

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