Update to slim3

[pierrejoye]

Before going wild, we should update to slim3 as the work will much easier without being live :)

Guidelines and changes: http://www.slimframework.com/docs/start/upgrade.html

Mainly the flash messages will be a slightly big changes.

[pierrejoye]

Now that slim 3 is out, refactoring/Update can be done, woohoo :)