Add JSON support:

 - rpc extensions to support both XML and JSON requests and responses
 - net support for a Eureka connection using JSON or XML
 - net support for marshalling and unmarshalling of the apps, app and instance structs
 - custom unmarshallers and intermediate wrapper structs to deal with nasty Eureka JSON
 - updated existing net tests to test both XML and JSON
 - added new sample-driven tests for JSON unmarshalling. (tricky to get that working--tests were a lifesaver)


I moved this up from the bottom of the file since it is closely related to GetApp. I also refactored it to just use GetApp so that I could reuse the unmarshaller. I think this is effectively the same by not overwriting current unless the retrieval was successful.