Make `_.escape` escape backticks as well

In Internet Explorer ≤ 8, the backtick character can be used to break out of unquoted attribute values or HTML comments. See http://html5sec.org/#102, http://html5sec.org/#108, and http://html5sec.org/#133.
Merge pull request #566 from mathiasbynens/backtick

Make `_.escape` escape backticks as well.