Added doModifyLimitQuery for SQLServer2012Platform that uses OFFSET... FETCH

SQL Server 2012 introduced new syntax for paging records using the OFFSET... FETCH clause. See http://technet.microsoft.com/en-us/library/gg699618%28v=sql.110%29.aspx for details on the specification.

This pull request:

* Adds doModifyLimitQuery specific to SQLServer2012Platform, using OFFSET ... FETCH instead of ROW_NUMBER() OVER(blah blah)
* Duplicates tests from SQLServerPlatformTest, using simpler OFFSET ... FETCH syntax

This version of doModifyLimitQuery will be much more robust than the one for SQLServer 2008 and below.
can't we just directly ` ORDER BY @@version` to skip evaluating outer `FROM` clause and skip `substr_replace`?