Update list of copyright holders, including contributors from 2012.

[[Imported from SVN: r357]]

COPYING

 Copyright holders:
 
-2011          Andreas Dedner
-2011          Jorrit Fahlke
-2011          Christoph Grüninger
-2011          Martin Nolte
+2012          Markus Blatt
+2011--2012    Andreas Dedner
+2012          Christian Engwer
+2011--2012    Jorrit Fahlke
+2011--2012    Christoph Grüninger
+2012          Robert Klöfkorn
+2012          Tobias Malkmus
+2011--2012    Martin Nolte
 2011          Elias Pipping
-2011          Oliver Sander
+2011--2012    Oliver Sander