[GH-252] Sqlserver limit

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

Url: https://github.com/doctrine/dbal/pull/252

Message:

Unfortunately fix from my last commit it not enough to handle more complicated queries.
This change should fix points 2 and 3 [from this list](http://www.doctrine-project.org/jira/browse/DDC-1785) but it also removes adding TOP statement if ``$offset`` is not set.
I know it looks realy messy but I can't figure out any other solution to handel queries with subqueries and ordering (such thing is generated after using Doctrine Paginator and ordering).