Support annotating additionalProperties field.

Added support for annotating the additionalProperties field.  Also added @JsonIgnore to this field by default to Jackson 1 and Jackson 2 annotators.

This change is required in scenarios where Jackson's field visibility has been turned up, causing the additionalProperties field to be serialized into the output.  Specifically, I was running into this problem when using POJOs from this project with Jongo.