Support parsing the ES6 'super' keyword.

Support parsing the ES6 'super' keyword.

This is basically implemented by imitating the handling of 'this', since they behave in very similar ways.  The keyword is defined in the various productions in §12.3 of the draft ES6 spec: http://people.mozilla.org/~jorendorff/es6-draft.html#sec-left-hand-side-expressions, and usage examples are available at e.g. https://github.com/lukehoban/es6features#classes.