Improvement of New Subscription form

- Use of more select2 dropdowns for billing account and plan version fields in New Subscription form.
- Cleans up existing subscription form view.
![screen shot 2014-03-10 at 7 15 07 pm](https://f.cloud.github.com/assets/716573/2380403/0a6c2982-a8aa-11e3-8085-9f5fe365c13e.png)