Adding fix in dealloc to remove text field observer.

The `UITextField` instance kept by `RMSTokenView` complained during `-[RMSTokenView dealloc]` because its observer was still attached while it was deallocated. This takes care of that.

Thanks for this library; it's sure coming in handy for me — and it's incredibly well-written. Which may just be a fancy way of saying it matches my coding style to a T (hah)!

You guys are awesome for providing this.