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.
Given that we know that offset and limit won't both be ``null`` (because of the early condition at the beginning), this could be simplified by doing:

```php
if ($offset === null) {
    $offset = 0;
}

$query .= " OFFSET " . (int)$offset . " ROWS";

if ($limit !== null) {
    $query .= " FETCH NEXT " . (int)$limit . " ROWS ONLY";
}
```

the initial condition could even be dropped because casting ``null`` to int gives ``0``