Add Refs API.

@christophercurrie
It's actually just a raw string which is the path appended to the https://orchestrate_host part. Here's an example:
```
curl -i "https://api.orchestrate.io/v0/$collection/$key/refs/?limit=$limit&offset=$offset&values=true"     -u "$api_key:"
HTTP/1.1 200 OK
Content-Type: application/json
Date: Wed, 18 Jun 2014 23:37:03 GMT
Link: </v0/ref_test/test_key/refs/?limit=1&values=true&offset=1>; rel="next"
Vary: Accept-Encoding
X-ORCHESTRATE-REQ-ID: 73721c80-f741-11e3-a1ad-12313d2f7cdc
transfer-encoding: chunked
Connection: keep-alive

{
  "count": 1, 
  "results": [
    {
      "path": {
        "ref": "fce549592ad8fe14", 
        "key": "test_key", 
        "collection": "ref_test"
      }, 
      "reftime": 1403132098275, 
      "value": {
        "hello": "jeff"
      }
    }
  ], 
  "next": "/v0/ref_test/test_key/refs/?limit=1&values=true&offset=1"
}
```