Switch to using jsoncpp as our json library. This lets us pretty-print the json file by default, and lets users place comments in the json file.
We will now only re-write the file when the actual logical structure of the json object changes, not only when the serialization changes.
Unfortunately, this will remove any existing ordering of profiles, and make the order random. We don't terribly care though, because when #754 lands, this will be less painful.
It also introduces a top-level globals object to hold all the global properties, including keybindings. Existing profiles should gracefully upgrade.
