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)


That debug output could use a separate module that would default separately.

```
metadataLog := log.MustGetLogger("fargo.metadata")
log.LogLevel("", log.DEBUG)
log.LogLevel("fargo.metadata", log.INFO)
```

Might be overkill, but it'd be good to make it easier to turn on Debug logging without the super-verbose metadata spam.