Moved all non-static methods in a separate class SimpleMapperService

The methods in SimpleMapper can be used static as they are, if the input is
presented in form of an instance of JsonParser.

An instance of SimpleMapperService can be configured, to use convenient
methods that take the input in several forms. The SimpleMapperService has
to be configured with an instance of JsonFactory, that is used internally,
to construct the JsonParser from the given input.