Finish adding fraction support.

Also, assorted other improvements.
Moved handleFractions() to outside of Equation object.
Gave regexes their own dictionary in Equation.__init__() for the sake
of easy reading and editing.
Added better support for floats.
Added support for inferring a one where there is no coefficicent, though I
still have to do it for x^1 terms.
Add support for inferring one coefficient for first-degree terms.