Add support for variables

[thecardinelli]

The Eval function should also accept a hashmap of string -> float64 for passing variables and their values in. This should use a symboltable under the covers.

I will likely work on this feature.

[soniah]

This looks a bit hairy at the moment, I didn't merge it in, only your trig code.

[soniah]

This code has been pulled in - thanks!