consistent name for 'dir' type

the header says 'directory', but the body says 'dir'. the name should be consistent.

``` 
curl localhost:8080 -v
> GET / HTTP/1.1
> User-Agent: curl/7.32.0
> Host: localhost:8080
> Accept: */*
> 
< HTTP/1.1 200 OK
< Date: Sat, 06 Dec 2014 20:52:16 GMT
< version: 0
< type: directory
< path: /
< checksum: 380030
< Content-Length: 111
< Server: Jetty(9.2.z-SNAPSHOT)
< 
{
  "version": 0,
  "path": "/",
  "sessionID": -1,
  "type": "dir",
  "checksum": "380030",
  "children": []
}
```