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.
Good point - I know I had some reason for writing it that way, but I can't for the life of me remember why now.