[core-api] Added Utility Class to convert to/from JsonElements

  * Closes #159
[core-api] Added Tests for JsonConverter
[dml-code-generator] Complex Value Types now externalize to Json
[jvstm-ojb] Code Generator now uses ValueTypeSerializer to externalize Value Types
[jvstm-ojb] OJB Internalizers now use the ValueTypeSerializer to internalize value types
[jvstm-ojb] ResultSetReader now working for ValueTypes with multiple externalization elements
[jvstm-ojb] JDBCTypeMap now knows that complex value types externalize to Json
[jvstm-ojb] DO_States are no longer Serializable
[dml-code-generator] Always cast parametric types on ValueTypeSerializer
[dml-code-generator] ValueTypeSerializationGenerator cleanup