Respect global [UIView areAnimationsEnabled] state.

Hi! I suppose `TOMSMorphingLabel` should also respect the global `areAnimationsEnabled` flag of the `UIView` class so that setting the text like this:
````objc
[UIView performWithoutAnimation: ^{
     tomsLabel.text = @"new text";
}];
````
will actually set the text without animation as expected.

The same goes for the following:
````objc
[UIView setAnimationsEnabled: NO];
tomsLabel.text = @"new text";
````

As per documentation for [`setAnimationsEnabled:`](https://developer.apple.com/library/ios/documentation/UIKit/Reference/UIView_Class/#//apple_ref/occ/clm/UIView/setAnimationsEnabled:), if we disable animations, the currently running animation blocks will continue, but subsequent animation blocks will perform without animation. So checking the `[UIView areAnimationsEnabled]` state in the `isMorphingEnabled` getter seems reasonable enough - it won't affect the already started animations but will prevent new ones to start.