quick fix for keepass header hashing

i added the header hashing to allow opening in current keepass clients - could only test on keepassx 2 for mac, so further feedback is appreciated. https://github.com/fdemmer/libkeepass/issues/2
removed print, check for existing HeaderHash field

the print statement was unnecessary, prevent unexpected behavior if the HeaderHash field does not exist (i.e. when the file was created using an "old" keepass that does not use HeaderHash