add image pulling

this isn't correct:
```
$ curl localhost:4243/images/create -X POST -v
* Hostname was NOT found in DNS cache
*   Trying 127.0.0.1...
* Connected to localhost (127.0.0.1) port 4243 (#0)
> POST /images/create HTTP/1.1
> User-Agent: curl/7.35.0
> Host: localhost:4243
> Accept: */*
>
< HTTP/1.1 200 OK
< Content-Type: application/json
< Date: Tue, 09 Dec 2014 01:10:01 GMT
< Transfer-Encoding: chunked
<
{"status":"Downloading from http://"}
{"errorDetail":{"message":"Get http://: http: no Host in request URL"},"error":"Get http://: http: no Host in request URL"}
* Connection #0 to host localhost left intact
```