added command line option for integration

minor refactoring replacing boolean "differential" argument with a
FunctionType enum
minor bug fix around 0 numerators and gcf

constructor was calling gcf for 0 numerators, which is really
undefined.  added a comment to this effect and short-circuited calling
of gcf when numerator is 0.
added integrate function
unit tests for integrate
Merge pull request #32 from stepthom/adding-integration

Adding integration