- don't call has_access directly from template, pass a staff_access variable instead

* All access control logic is now in access.py
* It exports a single method for general use: has_access(user, object, action)
 - possible actions depend on object type (e.g. 'see_exists', 'enroll', 'staff')
* Removed DARK_LAUNCH feature flag--it is now the default behavior
* Replaced check_course with three separate more focused functions that use has_access

Minor things:
* note on using pdb in testing
* moved time parsing helper into timeparse.py
* x_modules now have a .start attribute (None if not in metadata)

* pass user to check_course
* if dark launch feature enabled, users with staff access to course
can see courseware before start date.  Students still can't.
* tests.
* Remaining: enrollment view has custom access control.  Need to check it.

Removed duplicate index on simplewiki_namespace. Re-added migration that was exposing South 0.7.4 bug.

This reverts commit 7273868e.

Changed check_course from a decorator to a regular function that throws exceptions. Much easier to understand now.

Fixed the errors that were breaking tests from unused simplewiki files and a half-migrated certificates module.