Added tests for CGVectorAngleBetween

The old test ran successfully even though the result was wrong - 3 simple tests to ensure the function works as intended.
fix for CGVectorAngleBetween

# use acos instead of cos to calculate the angle
# acos returns "nan" when the function is called with the vectors (1,1),(1,1). due to floating point issues some 1.0f values will be interpreted as "> 1.0f" which is not allowed to use in acos  => ugly fix
fixed missing sign for perpendicular test
CGVectorAngleBetween fixes

# division by zero error for vectors of lenght 0
# use of acosf instead of acos
CGVectorNormalize fix

division by zero error possible
the function is technically undefined for this vector - so maybe throwing an error might be a better solution?
for now returning (0,0) seems OK for me... if I ever run into this I might wish I had thrown an error here because debugging this might be a pain - so sorry future me!
Merge pull request #1 from vigorouscoding/master

CGVectorAngleBetween fix by vigorouscoding