Add the option to use the java-docker API instead of the command line.

We do development on Windows which does not currently have a Docker
command line client.  This makes it hard to use the original version of
the plug-in.  This commit allows the use of the
[docker-java](https://github.com/docker-java/docker-java) client instead.
The following configuration options are added:

- useApi -- boolean which indicates whether to use the API or the command
  line client.  The default value is `false`.
- serverUrl -- the URL for the docker server.  If the value is null then
  we use the default value.
- username -- the name of the Docker user if authentication is required.
- password -- the password of the user if authentication is required.
- email -- the email of the user if authentication is required.