RFR: Move all JSON serializers to blueflood-http

These are used only in blueflood-http. I am moving these to the blueflood-http package. If we really need these serializers across modules, we should think about making them their own package. Currently, that is not the case

More context: I was working on PR: https://github.com/rackerlabs/blueflood/pull/195 where I need to add a serializer for elastic search results. It seemed bad to add blueflood-elasticsearch dependency on blueflood-core.