replace data.json with cheshire

We currently use data.json 0.1.1. data.json 0.2.0 is the latest version, but is not backwards-compatible, so an app that depends on 0.2.0 breaks codecs.clj. We can update to 0.2.0, but then we'd probably need to make codecs work with both api's, since lots of folks still use 0.1.1. I'd rather just switch to cheshire - it's faster, and has a more stable api.