Removed the use of NSStringFromCGVector/CGPoint().

These functions do not exist on OS X and weren't really that useful anyway.
For clamp() and clamped() it no longer matters if the first value is smaller or greater than the second value.
Fixed compiler errors in unit tests.
Add unit tests for the recent changes in the Int and CGFloat extensions.
Add functions to CGPoint that allow arithmetic with CGVector values.
Merge branch 'swift' of https://github.com/raywenderlich/SKTUtils into swift