Backspace rework (was placeholder PR #7)

See PR #7.

This PR incorporates @rcdilorenzo's test changes, and fixes the issues mentioned in the original PR.

The bug where a token was highlighted when deleting the last character was fixed by adding a new delegate method to RMSTextField, `willDeleteBackward:`, that gets invoked before `deleteBackward:` does its work (and thus we can determine whether the last token should be highlighted or not).

The placeholder test is a little trickier. A UITextField's placeholder label doesn't seem to allow user interaction, so I can only test that the view is there and that the textField's placeholder property is getting set and unset as needed.