Updated the toolbar item to fit better in the profiler styles

- Remove the item from the toolbar when there is no message (consistent
  with the logger and swiftmailer items)
- Add the number of messages directly in the toolbar without hovering
- Update the item according to the refactoring done in Symfony 2.1
Merge pull request #116 from stof/profiler_styles

Updated the toolbar item to fit better in the profiler styles