Two doctypes were being output. Only output one.
We don't need to write out an extra doctype since we're writing out the entire HTML tree, not just the root node.
Hack to fix attribute order in clean-exercises.py.

Summary:
class and id should come before all the other attributes, even though d comes
before i alphabetically.

Test Plan: Ran it against exercises/*.html; separate diff forthcoming.

Reviewers: eater, alpert, john

CC: csilvers, peter

Differential Revision: http://phabricator.khanacademy.org/D2145
Fix attribute ordering for meta http-equiv too.
Special case href for a tags, too.
Make clean-exercises.py output a meta Content-Type tag.

It appears that lxml is sensitive to the order of the attributes when it
handles meta Content-Type tags.
Clean up the exercise files using the new cleaning utility. (II)
clean-exercises.py should output a newline at the end of each file.
Clean up the exercise files using the new cleaning utility. (III)
Output UTF-8, except for &nbsp; which we change back ourselves.
Clean up the exercise files using the new cleaning utility. (IV)
Fix local mode.
Merge branch 'i18n' of github.com:Khan/khan-exercises into i18n