Create field that can evaluate math expressions

After seeing yet another person asking how to do this, and realizing that I've had to do this in the past, I went out to see if I could find a decent GPLable expression library. I found one and implemented it. It's actually a rather nice one and the implementation is trivial.

I like this patch, though I think I would rather put the math library in CTools than in Views. Other stuff could use it.