Add type converters to the Gson builder for LocalDate, DateTime and Instant.

- rename gson property to gsonMarshaller
- defined getGsonBuilderWithCommonAttributes() that is used to initialize
  the GsonBuilder with attributes common to the marshall and unmarshall
  configuration.
- Declare LocalDateTypeConverter, DateTimeTypeConverter, and
  InstantTypeConverter and register them with the Gson parser used for
  marshaling and unmarshaling.
- Define getGsonUnmarshaller() that lazy loads the GSon instance used for
  unmarshalling
Merge pull request #1 from WFMP/type-adapters-for-joda-time

Add type converters to the Gson builder for LocalDate, DateTime and Instant