[PATCH] Fix attempt to get column by class property name

PULL REQUEST: https://github.com/doctrine/doctrine2/pull/27

After upgrading from a Doctrine 2.0 RC to final version, I noticed a failure when using doctrine orm:schema:create:

[Doctrine\DBAL\Schema\SchemaException]
There is no column with name '_id' on table 'subuser'.

Since this worked before the upgrade, I investigated and found a new line of code that was attempting to get a column using the class identifier property name (_id) rather than the mapped column name (user_id). Please see patch.