Use MultiJSON

MultiJSON can clean up all the funky old JSON library detection stuff in Sinatra::JSON, but maintaining backward compatibility with all the old json_encoder options made it very messy, so I split it off into a separate module. 

I'd love to see Sinatra::MultiJSON replace Sinatra::JSON entirely (perhaps taking its name). Or, I can make one module that falls back when MultiJson doesn't understand the specified engine (e.g. :to_json). Let me know if you'd prefer it to look different.