[WIP]  Added DBAL session storaged based on PDO Session storage page

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | 2.1
| Fixed tickets | #881

I added a quick reference to https://github.com/symfony/symfony/blob/2.1/src/Symfony/Bridge/Doctrine/HttpFoundation/DbalSessionHandler.php basing it almost completely on the PDO Session handler

I'm merging it into this branch, even though it's not mantained anymore, because this is where this option became available to use for session storage 
What are the differences here? Do we have any idea why we should pick one versus the other? I just realized that we're giving the user a choice, and choices suck unless we can help them decide :).

Also, I would love to put these into a numbered list and link them to the individual headers. Something like this (but with different language, I'm being lazy):

>
1. Storing via PDO
2. Storing via Doctrine's DBAL