[DBAL-563] Add general IDENTITY generator type support for sequence emulating platforms

This is a follow-up PR for https://github.com/doctrine/dbal/pull/428 as discussed in [DBAL-563](http://www.doctrine-project.org/jira/browse/DBAL-563) which adds IDENTITY generator type support for platforms that do not natively support identity columns but can emulate them via sequences.
This replaces the current workaround for PostgreSQL `SERIAL` columns in favour of a more general approach. With this patch now IDENTITY generator can also be used with Oracle which did not work before. Future platforms can also make use of this approach if necessary.