Handle raw strings

This is a change to the lexer state machine to allow using raw string literals, like (print r"$\mu = 4")

Adds a new RawString class to the choice of states. 

Biggest change is in the state machine "process(..." method. The current system is able do a simple char-by-char loop over the input, but in order to handle r" as the beginning of a raw string, the loop needs to be able to look-ahead one character and push r" together as a single token.

I'm sure there is a more elegant want to handle the look-ahead.