Add support for empty and default value serialization (filtering)

[cowtowncoder]

It would make sense to be able to prune empty (and default value) output when serializing.

[mkoegel]

Would be great to get this and also honor the SerializationFeature.WRITE_EMPTY_JSON_ARRAYS setting.

[cowtowncoder]

I will add a separate feature request for that, thank you for suggestion.

[cowtowncoder]

Adding isEmpty() methods is easy (and now done), but turns out that we need bit more elaborate support to make recursive serialization handling work as expected. Problem being that exclusion is "off by one", that is, calling BeanSerializer checks inclusion. But object/array serializer also needs configuration information for content; and this is where things get interesting.
Due to timing, can not get this done for 2.6, but may revisit for 2.7. Doing this also allows considering similar support for jackson-databind JsonNode, which should be nice; we can use this package as sort of prototype or dry dun.