Respect global [UIView areAnimationsEnabled] state.
Invoke the completion block even if setting text without animations.
Use a separate method to check both isMorphingEnabled and [UIView areAnimationsEnabled]
Break the strong reference cycle between CADisplayLink and TOMSMorphingLabel.

CADisplayLink retains the object set as its target and TOMSMorphingLabel references the display link strongly. This gives a strong reference cycle and TOMSMorphingLabel is never deallocated. This commit adds a helper class named TOMSMorphingLabelWeakWrapper which holds a weak reference to the label and forwards necessary messages to it. This breaks the reference cycle and allows deallocating the label.

Note that if the weak wrapper's label reference gets nullified due to the label being deallocated, the display link would continue firing its events to the weak wrapper. So it is necessary to properly invalidate the display link in the TOMSMorphingLabel's dealloc.
Merge branch 'fix/completion-block-not-animated' of https://github.com/wanderwaltz/TOMSMorphingLabel into 0.5
abstracted execution of text`s completion block
Merge branch 'feature/uiview-performwithoutanimation' of https://github.com/wanderwaltz/TOMSMorphingLabel into 0.5
bumped version to 0.5 and added documentation for it; minor cleanup;
Merge branch '0.5'