Remove ZERO_WIDTH_SPACE_STRING, relying on iOS 6 to do it right.

As of iOS 6, a UITextField subclass will receive -deleteBackward
even when the text field is empty. So ZERO_WIDTH_SPACE_STRING is
no longer needed there.
Add a delegate method to notify when the text field text has changed.

This is simpler than requiring the client to call -addTarget:action:...
on our textField.
Align tokens on the last line vertically with the text field.

If the text field is larger than the JSTokenButton images, the
buttons may appear to float higher than the text, which looks
odd and distracting.

Note that this means that when the text field wraps to a new line,
the previous line's tokens may shift vertically to fit more compactly
with each other, if the text field is taller than the tokens.
Merge pull request #17 from bjhomer/pull-requests/ios-6

Remove ZERO_WIDTH_SPACE_STRING, relying on iOS 6 to do it right.
Merge pull request #18 from bjhomer/pull-requests/token-field-text-did-change

Add a delegate method to notify when the text field text has changed.
Merge pull request #19 from bjhomer/pull-requests/align-tokens-vertically-with-text-field

Align tokens on the last line vertically with the text field.