Add a JacksonMatcherFilter.

This new module and class combines the jackson json filter with a Matcher to output only the json that is requested.

Fixes #4
The class is actually implicitly _not_ thread safe. You need to use one per thread, which you get for free because the use of ObjectMapper (where this class is used) is also not thread-safe so you have to create an instance for each thread (and usually for each serialisation request too).