move COPYRIGHT_YEAR to clientversion.h

- this allows usage of that information also in other places (e.g. splash
  screen)
use the new COPYRIGHT_YEAR macro in bitcoin-qt.rc

- this reduces the places, where we need to change the year further
fix bitcoin-qt.rc not showing up in Qt Creator
Merge pull request #2503 from Diapolo/copyright-year

new COPYRIGHT_YEAR macro in clientversion.h