OSX startup fixes

This is a second attempt to get the pwd on OSX fixed, following my remarks in rev. 2ad6afbb. It fixes up the gui_path and makes sure that we launch the engine in the user's homedir and not in some random subdir of the app bundle. I think that this is the most sensible way to do it, and it matches the behavior on the other platforms, making it easier for the user to figure out the cwd in which the program runs.

Tested on Mac and Linux. @jwilkes One thing I'm not sure about is whether the process.cwd() function which I use there to determine the gui_path exists in nw.js 0.14.7 on Windows. I'd say that it's a safe bet, and in any case it shouldn't really matter since that code path only executes on OSX. But if you're getting hiccups in the startup on Windows then that might be the cause, and I'll have to fix it, so please let me know if you're seeing any issues there.