Bug fix for normalized() 

when self == CGPoint.zeroPoint, a call to normalized returned (nan, nan) so I added a check for when length() == 0
bug fix for normalized()

normalized() returned (nan, nan) if length()==0
Merge pull request #7 from icanzilb/swift

normalized() on CGPoint and CGVector now returns zero instead of NaN when the length of the vector is zero.