[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 54 15 pm](https://f.cloud.github.com/assets/4573957/791725/668ba9a0-eb4e-11e2-8048-00d1c222c3f1.png)