Added support for \not

Addresses issue https://github.com/Khan/KaTeX/issues/29.  Requires a Huxley test which I will try to add tomorrow.  Here's a screenshot:
![screen shot 2014-09-30 at 9 31 54 pm](https://cloud.githubusercontent.com/assets/1044413/4469952/182b2cea-491c-11e4-8274-4f44fccfa05a.png)

This makes an assumption that group.value.body is a "rel".  It really could be anything else.  LaTeX and MathJax allow this.  What they end up doing is placing the combining long solidus overlay glyph infront.  Because this glyph already includes metrics that cause it to be rendered over the previous glyph, producing similar behaviour in these edge cases will require extra work.