[TITokenField] Fixed issue where addToken: does not get rid of placeholder text.

Using addToken: directly without typing any text now gets rid of placeholder in TITokenField.

Fixes this:
![screen shot 2013-07-12 at 4 41 43 pm](https://f.cloud.github.com/assets/4573957/791720/f46a4228-eb4d-11e2-9dcb-82198d60f018.png)