Handle caps in service configuration

This handles the following format for caps specified on services:

``` yaml
service:
    caps:
        add:
            - SYS_NICE
        drop:
            - NET_ADMIN
```

Here's where --cap-add and --cap-drop were added to docker:
https://github.com/docker/docker/pull/6968

Also this feature has been added to docker-py:
https://github.com/docker/docker-py/pull/314

To confirm caps in a container:

```
apt-get install libcap-ng-utils
captest --text
```

MISSING: New docs. New tests. So far none of the tests have failed but I am still waiting on a few.