Handle postgres sequences

Hi,

I added the code in the existing, but empty, file Zend/Db/Adapter/Driver/Pgsql/Feature/SequenceHelper.php + unit tests.

You can find usage examples there https://github.com/geelweb/zf2-db-tests, I tested 3 use cases : 
 * sets the sequence name to getLastGeneratedValue()
 * sets the sequence name to the tableGateway with setSequence()
 * use the features MetadataFeature + SequenceHelper to find the sequence name