Increasing the number of open files by docker process.

With the more extensive/intensive use of docker on day-to-day activity, this error might occur:

```bash
$ tail -f /var/log/docker.log
2014/07/02 20:40:15 http: Accept error: accept unix /var/run/docker.sock: too many open files; retrying in 1s
2014/07/02 20:40:16 http: Accept error: accept tcp [::]:2375: too many open files; retrying in 1s
```

This pull request has the goal of increasing docker's open file limit, avoiding this error.
I'm +1 on this change, but can we please use the limit values that upstream does for consistency?
https://github.com/docker/docker/blob/master/contrib/init/openrc/docker.initd#L14-L15
https://github.com/docker/docker/blob/master/contrib/init/systemd/docker.service#L9-L10
https://github.com/docker/docker/blob/master/contrib/init/sysvinit-debian/docker#L91-L96
https://github.com/docker/docker/blob/master/contrib/init/upstart/docker.conf#L5-L6