Added folloing methods:

- (void)centerInRect:(CGRect)rect;
- (void)centerVerticallyInRect:(CGRect)rect;
- (void)centerHorizontallyInRect:(CGRect)rect;
- (void)centerInSuperView;
- (void)centerVerticallyInSuperView;
- (void)centerHorizontallyInSuperView;
- (void)centerHorizontallyBelow:(UIView *)view padding:(CGFloat)padding;
- (void)centerHorizontallyBelow:(UIView *)view;
:
Added an import to stop breakage
Added a set of methods inspired by Kevin O'Neill's UsefulBits UIView+Positioning methods
Updated podspec