[feature] introducing JSON mapper

the JSON mapper is a new service the scim-schema offers its clients. the
SCIM spec notices only JSON as serialized representaion of resources.
though it might be technical possible to map SCIM resources to other
data formats, e.g. XML or YAML, the main data format for scim resources
stays JSON. this fact should also manifest itself by the scim-schema
offering JSON <-> POJO mapping facilities for its clients. before the
introduction of JSON mapper only (de-)serializers and annotated classes
have been provided. both had a hard dependency on Jackson, which might
not be the best idea, at least when thinking about stripping down
dependencies. JSON mapper should serve as a higher value, more concrete
JSON <-> POJO mapping facility for SCIM resources without published
dependencies on Jackson.