Fixed derivative control and added sharp turn case

Derivative control didn't make sense since we were setting preverror =
error when preverror != error then doing kD * (preverror - error). In
all cases, derivative feeback would have been 0.  Also added a
"hardcoded" wheel speed that tells a wheel to go slower if we're way off
the tape. We should look to making new_line_following.pde more
encapsulation friendly. i.e. we can dump all its contents into a
different header or c++ file so our main control logic won't be so
crowded.