Added public_trial_statement field to the public_trail edit form.

This PR fixes the issue #448.

![bildschirmfoto 2014-08-05 um 08 11 32](https://cloud.githubusercontent.com/assets/485755/3807471/68a951d8-1c67-11e4-8e1e-40e9aee7c0b5.png)

@lukasgraf could you have a look?