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)


This was causing me problems, and so I was wondering what its purpose really was. Is it just to make sure that the registration stuck? Or is it reading back real values that Eureka has provided? 

If the former, maybe just checking the err and rcode from getBody() would be enough. If the latter, I think it should use the same trick that readAppInto() uses.