Fix #18 Memory Leak in operator=

- free internal variables before assigning new values
- used a reuseable private member function now
- first report by Ahnfelt and further investigation done
  by Paul
- see http://sourceforge.net/p/pngwriter/discussion/238247/thread/c7607b9b/
Merge pull request #23 from ax3l/fix-memoryLeakAssignOp

Fix #18 Memory Leak in operator=