I ran into this problem today also. It's annoying to see an output of 0x73 for your forward motor and then 0xFFFFFF8C for the backward one (I'm printing in hex because I am debugging a program that was printing in binary and I was looking at the output of that program with a hex editor)
I think if the sign extension was gotten rid of by using reinterpret_cast<unsigned long> rather than (unsigned long) before calling printNumber (in Print.cpp) it would work better.