Added a C code printing method for the sign function.

Is there a better way to specify specific printing for a function that doesn't map to a keyword in the target language? This sign function implementation is also valid in Python and maybe some other languages too. Also, I'm not sure why C doesn't have a sign function in the math library or something. Here is a Stackoverflow showing some implementations: http://stackoverflow.com/questions/1903954/is-there-a-standard-sign-function-signum-sgn-in-c-c
This should be `pow(x, 2)` not `x**2`.