Add support for getter methods in object schema generation

[wuan]

Currently, the schema generator only supports public fields when determining the object schema. This behaviour should be expanded to public getter methods.

[uweschaefer]

+1

[JoergAdler]

Better we take the @JsonAutoDetect-Annotation into account and then determine if getters and/or setter or fields would be in the schema. (Maybe with the help of a standard-plugin)