Allow setting and getting of resource class on Rack

Fixes: #29

### Create with Resource Class
```json
curl -vX POST -H 'Content-Type: application/json' -H 'Accept: application/json' -v -d  '
{
  "subnet": "192.168.1.0/255",
  "name": "my_rack",
  "capacities": [{
    "name": "total_cpu",
    "value": "64"
  }, {
    "name": "total_memory",
    "value": "1024"
  }],
  "nodes": [{
    "id": "123"
  }, {
    "id": "345"
  }],
  "slots": 1,
  "resource_class":{
       "id":1,
       "links":[
          {
             "href":"http://0.0.0.0:6385/v1/resource_clases/1",
             "rel":"self"
          }
       ]
    }
}
' http://0.0.0.0:6385/v1/racks
```