Add a feature in `JaxbAnnotationIntrospector` to define meaning of `nillable=false` as "JsonInclude.NON_EMPTY"

[cowtowncoder]

(see #48, #49 for background)

Meaning of nillable property of @XmlElement and @XmlElements comes from XML Schema, and defines whether it is legal/mandatory to suppress writing out of empty collections. For Jackson this can be achieved by returning different inclusion criteria for a property annotated with one of these annotations.

For a longer discussion, see the original change #39 (for Jackson 2.5.1), which covers many aspects of handling.

Since I am not convinced that the change suggested in #48 / #49 is safe (although perhaps returning Include.NON_NULL might be acceptable, more so than NON_EMPTY), I will add a setting with which one can specifically define which JsonInclude.Incude value to use for nillable = false; default being null (meaning none). To do what #48 asks, Include.NON_EMPTY should be returned, or possibly Include.NON_NULL.

[cowtowncoder]

Adding method setNonNillableInclusion in JaxbAnnotationIntrospector, as well as JaxbAnnotationModule; either way, needs to be called before introspector is registered with ObjectMapper.

To implement behavior desired for #48 / #49, should set value either as JsonInclude.Include.NON_NULL or JsonInclude.Include.NON_EMPTY, depending on exact desired semantics: former will not write property if value is strictly Java null; latter if it is either Java null OR value defined as empty, most commonly Collection or Map with size of 0.