[WIP] ObjectMapper reuse & config

I had a pressing need for this capability. We are using finatra 1.5.3, and I merged the latest master branch into this one. Aside from that, the `ResponseBuilder` constructor takes a serializer with a default argument `DefaultJacksonJsonSerializer`. Based on @davemssavage's recommendation, `DefaultJacksonJsonSerializer` is an object. Let me know what you think. I would love to see this merged into the next finatra release.