dockerio.pull broken on verified images since docker 1.3

Hey Guys

Pulling docker verified images (which is new in docker 1.3) is broken if the image already exists locally, for example (using the pydsl):

``` python
state('.nginx-image').docker \
    .pulled(
        name='nginx',
        tag='latest',
        user='root') \
    .require(stateconf='docker::goal')
```

Returns this error (when the image exists)

```
----------
          ID: nginx::nginx-image
    Function: docker.pulled
        Name: nginx
      Result: False
     Comment: An error occurred pulling your image

              {"status":"The image you are pulling has been verified","id":"nginx:1.7.6"}
              {"status":"Already exists","progressDetail":{},"id":"511136ea3c5a"}{"status":"Already exists","progressDetail":{},"id":"5a7d9470be44"}{"status":"Already exists","progressDetail":{},"id":"feb755848a9a"}{"status":"Already exists","progressDetail":{},"id":"7caab58ebef6"}{"status":"Already exists","progressDetail":{},"id":"847cf46d3d69"}{"status":"Already exists","progressDetail":{},"id":"ddd54642be42"}{"status":"Already exists","progressDetail":{},"id":"ad5b7af53616"}{"status":"Already exists","progressDetail":{},"id":"44a3dbb90234"}{"status":"Already exists","progressDetail":{},"id":"ed12dc3482fc"}{"status":"Already exists","progressDetail":{},"id":"7f9cd959dc81"}{"status":"Already exists","progressDetail":{},"id":"cf2c83853506"}{"status":"Already exists","progressDetail":{},"id":"2e980420641d"}{"status":"Already exists","progressDetail":{},"id":"b8c5a6ffc2bd"}{"status":"Already exists","progressDetail":{},"id":"f3f21bfe3d94"}{"status":"Status: Image is up to date for nginx:1.7.6"}
     Started: 10:09:53.022459
    Duration: 291965.096 ms
     Changes:
```

Digging around the code I think its an issue with this line: https://github.com/saltstack/salt/blob/2014.7/salt/modules/dockerio.py#L1499 which explicitly checks for the string to contain `Download complete`, for verified images docker will now return `Already exists` where as none verified images will still return `Download complete` even though they already exist.

This results in `infos` returning `None` which results in the pull method returning an error even though no error occurred.

I might try and get a PR in for this if I get some time over the weekend.