Human readable description support

See: https://github.com/FasterXML/jackson-module-jsonSchema/issues/12

JSON Schema (v3 and v4) allows a human readable description to be associated with each field: http://tools.ietf.org/html/draft-zyp-json-schema-03#section-5.22

It would be good to have an annotation for defining a description for each property, which the JSON Schema Jackson module can respect.