Started fraction.
Can handle any int combination, and will output fraction in lowest
terms.
Also added a to do list, and a simple solver interface for terminal
testing for now.

Also. currently, cannot handle 2 minuses, aka 2 - -2, 2 - -2.
This causes it to go past the stack.
Can fix by either looking though the stack for it, and converting any
two minuses in a row into a plus, or just making it "-2" rather than
"-", "2" in the shunting algorithm.