show trial message as pop up instead of a django message

![screen shot 2014-05-06 at 1 23 49 pm](https://cloud.githubusercontent.com/assets/716573/2892979/68356edc-d543-11e3-8e79-f7e487f1b005.png)

Also adds:
- trial default plans `is_trial` added to `DefaultProductPlan`
- Advanced trial is set up in migration for this...