FileDir::getApplicationDir() implementation for OSX

This patch adds missing code needed to run opencaesar3 on OSX. There is still one problem to solve: the game won't compile with clang because of smart pointers + pimpl combination (see compilation log: http://pastebin.com/ZB1ViGNC). Apart from this everything should work fine now. I was able to play for a little bit before the game crashed.