Add a feature should clear the input every time

When I click "Add a feature" after adding a feature, the dialog pop up box still have the feature I added last time.

Ideally, the input fields should be cleared right after a feature is successfully added.

![2013-05-01 00 53 37](https://f.cloud.github.com/assets/1693418/448148/6f0c203a-b234-11e2-846d-422b364bc29e.png)

#cs169test