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)

It seems a little weird having a symbol with the same name as a function.  The upside, is that functions are parsed first which effectively hides this symbol which is nice because it shouldn't be used in isolation.