Rename DockerClient to Client and keep DockerClient as is.

Since the returned interface doesn't behave exactly like DockerClient
did, it's better to have an explicit Client interface and keep
DockerClient the implementation.

People wanting to use the mock interface will have to alter their code
to accept the interface.

Signed-off-by: Andrea Luzzardi <aluzzardi@gmail.com>
Merge pull request #31 from aluzzardi/mock-interface

Rename DockerClient to DockerClientInterface and keep DockerClient as is...