* add possibility to configure custom scalar types externally

[kuptservol]

1. Added possibility to configure custom type to GraphQLScalarType resolvers.
   To add resolver you must implement interface com.oembedler.moon.graphql.engine.dfs.GraphQLTypeResolver and add it to configuration with addGraphQLTypeResolver method
2. Do not try to expose private final static contstants - cause they are private - commonly it is unreasonably to expose to client Serializable serialVersionId