Improve the batch beta tester add feature

Includes:
* Allow batch beta enrollment by username
* Add "Auto Enroll" for bulk beta tester add
![screen shot 2014-04-14 at 11 04 49 am](https://cloud.githubusercontent.com/assets/1985317/2696777/a5f1016e-c3e6-11e3-8fdc-4616d721bd2e.png)

* Both batch enrollment & beta test -- clear text input box after clicking "Enroll"/"Add beta testers" or "Unenroll"/"Remove beta testers"
*  Both batch enrollment & beta test -- reset 'Auto Enroll' and 'Notify users by email' checkboxes to default "checked" state after clicking "Enroll"/"Add beta testers" or "Unenroll"/"Remove beta testers"

![screen shot 2014-04-14 at 11 06 37 am](https://cloud.githubusercontent.com/assets/1985317/2696778/a5f1367a-c3e6-11e3-9e16-cd45b4c97817.png)