add syntax highlight via codemirror to the examples on the pyret.org site

@jpolitz I added support for syntax highlighting using codemirror. I have it _not_ highlighting other languages, but I didn't see an easy way for it not to try to highlight the couple non-code pre's in the tour (ie, the results of running a program, error messages, etc). Overall I think it is worth it (the front page looks a lot nicer), but up to you if you want to merge, build, and push.