In UIView, [super init] is exactly equal to [super initWithFrame:CGRectZero].
Move UIResponder overrides into a class extension
Clarify and expand logic for determining if backspace has been tapped

Inspired by JSTokenField.
Make text property readwrite

Inspired by JSTokenField.
Revert "Remove backingView property"

This reverts commit 4424c15647d87f09a99a6d6f16cbd6bd2b0b13c8.