3.0 session

This changes Sessions to not be static anymore and puts a setter/getter inside the request object as discussed in https://github.com/cakephp/cakephp/issues/2560

I have not yet changed the method names to what was proposed in that ticket as I wanted a review on these changes first.

With this merged, we have no longer need for running tests with `--stderr` and we can safely use phpunit 4.1