Update build instructions for Windows.

Use VS2013; remove path to qtrepotools, which only exists
in git checkouts.

Task-number: QTBUG-40421
Change-Id: I6409888920041ca1146ee8025aba7ed8aa9c91ad
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>