[#21] Teacher can see when a session is already running

Relative issue: #21
Instead of having a simple status displayed in *ChooseActivityFlowDialog* layout, I strengthened a little bit this step by locking the "use prepared questions" button when a session is already running.
Here is a preview of the feature:
![screen shot 2014-04-08 at 22 38 32](https://cloud.githubusercontent.com/assets/3521208/2649078/bc483482-bf5e-11e3-8244-1e992680593b.png)