[#40] After a reset, there is no more "Session already running" popup

This pull request fixes the following issue : https://github.com/SMILEConsortium/smile_teacher_android/issues/40 

Instead of having the popup "Session is already running", if there is a session, I just show a toast "recovering...". 
![recovering](https://f.cloud.github.com/assets/3521208/1552794/9d77fe44-4e29-11e3-85c2-5c51cff4f89c.png)


Else, if it is a reset, it will show "Creating...".
![creating](https://f.cloud.github.com/assets/3521208/1552792/9194c9a4-4e29-11e3-91d4-cb9585aba3a6.png)