Enable/Disable vsync as a flag to init

[thebracket]

Let the client determine whether or not they want vsync, via a flag. See rltk.cpp line 34.

[Autsider666]

I've added a function to toggle framerate to rltk.cpp, maybe also useful for other people?
http://www.sfml-dev.org/documentation/2.0/classsf_1_1Window.php#af4322d315baf93405bf0d5087ad5e784

Edit: nvm, both features are easily done with get_window() so it's not really hard to do atm.