Redirect daemon output to /var/log/docker.log

when somebody wants to play inside the same container where dind is running, can do:

```
docker run --privileged -it jpetazzo/dind
```

which starts docker in the background, and spawns a `/bin/bash` which is fine,
bu docker daemon log and docker client command output is mixed:

```
# docker images
2014/06/26 10:46:05 GET /v1.11/images/json
[87dddcd3] +job images()
[87dddcd3] -job images() = OK (0)
REPOSITORY          TAG                 IMAGE ID            CREATED             VIRTUAL SIZE
```

with the proposed changes docker daemon'g log would go to `/var/log/docker.log`
making a clean client output:
```
# docker images
REPOSITORY          TAG                 IMAGE ID            CREATED             VIRTUAL SIZE
```
make docker daemon log to stdout (default) or file depending on LOG env var
Merge pull request #23 from lalyos/patch-1

Redirect daemon output to /var/log/docker.log