Add of dismissible class

Hi

I added an extra class to ensure that long messages are well displayed. It is a standard class of "Twitter Bootstrap."

Kind regards
Maarten

--- Before
![before](https://cloud.githubusercontent.com/assets/849003/3965656/a2004bd4-2799-11e4-9fd2-a71e7532f897.png)

--- After
![after](https://cloud.githubusercontent.com/assets/849003/3965657/a23f04dc-2799-11e4-819f-de33fe0a55ac.png)