Added JsonPdxConverter (#2)

Added JsonPdxConverter (#2)

- Allows PdxInstance objects to be converted to JSON bytes when sourced
from a Geode region into a Kafka topic
- Allows JSON bytes to be converted to PdxInstance objects when sinked
into a Geode region from a Kafka topic
- Added unit and DUnit tests for JsonPdxConverter
- Added functionality to the test framework to specify custom key
converter, custom value converter and configuration properties for each
- Added TestObject class to allow validation of
serialization/deserialization

Authored-by: Donal Evans <doevans@pivotal.io>