[WIP} Added deadlock check

I've added the interface method and drivers implementation to check if
a given exception is a deadlock so can be retried by the pull request
on ORM https://github.com/doctrine/doctrine2/pull/806

At the moment I've only implemented MySql checks.

I'm not sure about where to place the tests, the only I've found about Drivers are those regarding OCI8. Where should I put those?