Rename current_user to current_visitor

- Rename current_user to current_visitor.
Merge branch into pull_request_02_19_2013_a

- Merge 'rename_current_user_to_current_visitor_02_15_2013_a' into
  'pull_request_02_19_2013_a'
Fix feature test for left pane navigation

- Fix feature test for left pane navigation.
Merge branch into pull_request_02_19_2013_a

- Merge branch 'rename_current_user_to_current_visitor_02_15_2013_a'
  into 'pull_request_02_19_2013_a'
Merge zeus/pull_request_02_19_2013_a into zeus_updates

This test was fixed in master before the merge

Conflicts:
	features/teacher_left_pane_navigation_state_should_be_maintained.feature