Added definition for Handlebars.Utils.escapeExpression()

It fixes https://github.com/borisyankov/DefinitelyTyped/issues/592
I think Utils is not class, it module. https://github.com/wycats/handlebars.js/blob/4f01f650dc2458343fcb40147803b30ba9a724de/lib/handlebars/base.js#L1