Add delegate method to disallow deleting tokens.
Don't animate the frame around if the view isn't on screen yet.

This was causing a problem when the token field was in a view that
was presented modally. `layoutSubviews` was called after the view
was added to the window, but before it had any original frame. Thus,
the animateWithDuration call here was causing it to animate from
CGRectZero, which looked odd.
Merge pull request #23 from bjhomer/pull-requests/should-remove

Add delegate method to disallow deleting tokens.
Merge pull request #24 from bjhomer/pull-requests/dont-animate-when-not-visible

Don't animate the frame around if the view isn't on screen yet.
Improve how token field handle Backspace and highligh token

Highlight token only when text field has NO text and user press backward (rather than the text field have ONE character and user press backward)
Check respondsToSelector before calling an @optional delegate method.

Oops.
Merge pull request #26 from bjhomer/pull-requests/behavior-fixes

Bug fixes.