Changes related to Phil's changes to interstitials and feedback:
- Switched Turtle to use new interstitials and feedback.
- Moved common messages to common.soy.
- Generated en_us files.
- Loosened duplicate message checking in xliff_to_json.py.
Merge remote-tracking branch 'codedotorg/mooc' into mooc

Conflicts:
	i18n/common.pyc
	static/maze/generated/en_us.js
	static/turtle/generated/en_us.js
	static/turtle/template.soy
Regenerated files after merge.
Fixed change preventing application-specific errors.
Added msg tags for interstitial messages in Turtle.
Made all of the Turtle feedback work with the new system, with the
exception of a bug displaying feedback after Level 9 of Turtle 3.
Fixed post-interstitial bug and cleaned up code.
Merge remote-tracking branch 'codedotorg/mooc' into mooc

Conflicts:
	static/common.js
	static/extracted_msgs.xlf
	static/maze/generated/en_us.js
	static/maze/maze.js
	static/maze/template.soy
Fixed poor merge.
Merge remote-tracking branch 'codedotorg/mooc' into mooc

Conflicts:
	i18n/xliff_to_json.py
Incorporated Nan's helpful feedback.
Merge remote-tracking branch 'codedotorg/mooc' into mooc
Renamed draw_{move,turn}_inline and to draw_{move,turn}_by_constant.
Cleaned up a little code in template.soy.
Removed "foo".
Merge pull request #47 from espertus/mooc

Bringing Turtle up-to-date.
merged en_us.js