Use jsoncpp instead of boost property_tree

[sharkwouter]

Since I'm trying to see if I can get this game to work on Vita or PSP, I had to remove the dependency upon boost. Instead I'm using jsoncpp to track the configuration file now, which is working well. Maybe it is useful to you as well, but feel free to close this if it isn't.

[sharkwouter]

Actually, I'll come up with a differet approach. Jsoncpp is too much trouble on Vita. It works fine on PSP, though. I would have gotten panel-pop fully running on PSP now, but their SDL2 port is kind of broken, so I have to fix that first.

[sharkwouter]

I've since gotten jsoncpp working on the Vita, so I'll reopen this.