Pluggable JSON Libraries

A potential solution to #45 (it also increases test coverage to ~81% which benefits #76).
Basically, I removed most of the old json wrapper. Instead Finch users must implement either implicits or filters to convert their preferred Json representation to something that Finch can send in an HTTP response. 

As examples, the implicit for the scala 2.10 library is included in the updated docs.
The implicit conversion for argonaut is similar. It basically just involves calling `asJson` instead of `toString` inside the implicit (after you have setup the implicit conversions for argonaut, of course).
Jackson is a bit more complicated, but basically involves invoking an object mapper inside the implicit.
Because the nature of the conversion can be very dependent on the nature of the data (depending on library choice), I've left the conversion itself to Finch users to implement.

I would like any thoughts on this PR. I'm not sure its exactly what Finch needs for pluggable json libraries, but I think its a step in the right direction.
Is it possible to get rid of this `JsonResponseConverver` but having an implicit converter imported? It guess scala compiler will figure out that it has to use an implicit converter and return not `JSONObject` but `Json`. Could you please check it?