backends: add fleetdocker

proof of concept that implements the dockerserver interface on top of fleet. This is mostly me exploring how swarm works but maybe someone will find it fun.

```
./swarmd 'dockerserver tcp://localhost:4243' 'debug' 'fleetdocker'
```

```
core@core-01 ~ $ DOCKER_HOST="tcp://localhost:4243" docker ps
CONTAINER ID        IMAGE               COMMAND             CREATED                  STATUS              PORTS               NAMES
docker-81863        busybox             [/bin/sleep 5000]   Less than a second ago   Up                                      docker-818639ac48a4c6af.service
core@core-01 ~ $ DOCKER_HOST="tcp://localhost:4243" docker stop docker-81863
docker-81863
core@core-01 ~ $ DOCKER_HOST="tcp://localhost:4243" docker ps
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS               NAMES
core@core-01 ~ $ DOCKER_HOST="tcp://localhost:4243" docker run busybox /bin/sleep 5000
2014/06/09 09:18:33 EOF
core@core-01 ~ $ fleetctl list-units
UNIT				STATE		LOAD	ACTIVE		SUB		DESC			MACHINE
docker-f1581a8b9525e20f.service	launched	loaded	deactivating	stop-sigterm	f1581a8b9525e20f	cc6eef37.../10.0.2.15
core@core-01 ~ $ DOCKER_HOST="tcp://localhost:4243" docker ps
CONTAINER ID        IMAGE               COMMAND             CREATED                  STATUS              PORTS               NAMES
docker-f1581        busybox             [/bin/sleep 5000]   Less than a second ago   Exited (0)                              docker-f1581a8b9525e20f.service
core@core-01 ~ $ DOCKER_HOST="tcp://localhost:4243" docker stop docker-f1581
docker-f1581
core@core-01 ~ $ DOCKER_HOST="tcp://localhost:4243" docker ps
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS               NAMES
core@core-01 ~ $ fleetctl list-units
UNIT	STATE	LOAD	ACTIVE	SUB	DESC	MACHINE
```