[GH-249] Fixed doModifyLimitQuery for SQLServerPlatform

This issue is created automatically through a Github pull request on behalf of norzechowicz:

Url: https://github.com/doctrine/dbal/pull/249

Message:

It seems that there is a problem in SQLServerPlatform ``doModifyLimitQuery`` method.
I can't use Doctrine ORM Paginator because of wrong position of ``DISTINCT`` statement in final limit subquery, here is the exception message I'm getting.
``SQLSTATE[42000]: [Microsoft][SQL Server Native Client 10.0][SQL Server]Incorrect syntax near the keyword 'DISTINCT'.``

My proposition is to change ``doModifyLimitQuery`` result from:

``SELECT * FROM (SELECT ROW_NUMBER() OVER (ORDER BY username DESC) AS doctrine_rownum, * FROM user) ...``

into:

``SELECT * FROM (SELECT *, ROW_NUMBER() OVER (ORDER BY username DESC) AS doctrine_rownum FROM user) ...``