first pass at avoiding under/overflow on subtract and add operations for ofColor
implemented clamping for all basic mathematical operations on ofColor, expanded CLAMP macro to avoid multiple evaluation
rewrote normalize() to use operator/=()
removed template specializations for ofColor_<float> by making all the clamped arithmetic operations into methods of ofColor, allowing them to access limit()
moved from .h back to .cpp as functions, less code this way
Merge pull request #2237 from kylemcdonald/fix/ofColor-underflow

Fix ofColor overflow/underflow