Cleanup / remove conditionals to switch between GMP and BC

So this is what I meant on #13 's thread. 

If you look at EcDH and CurveFp, I removed all direct references to either bc or gmp and extracted the function calls to two new classes (oddly named Gmp and BcMath :D ).

If you guys think this solution is OK, let me know and I will do the same to all other classes that have those if's laying around...