[DBAL-81] Add possibility to set the auto-commit mode for a connection

This PR makes auto-commit mode configurable for a connection (as proposed in [DBAL-81](http://www.doctrine-project.org/jira/browse/DBAL-81)).
The implementation adds methods to get and set the auto-commit mode on a connection object. When NOT in auto-commit mode, each `commit()` and `rollBack()` call automatically start a new transaction after the operation is finished. Aditionally a call to `connect()` starts a new transaction if not already connected.
When IN auto-commit mode, the behaviour is the same as it is with the current implementation.
When switching auto-commit mode, all current active transactions are committed.