Dm plugin status

Add driver name and status details to docker info output

Here is some example output:
```
# docker info
Containers: 7
Images: 9
Driver: devicemapper
 Pool Name: docker-0:31-15505662-pool
 Data file: /var/lib/docker/devicemapper/devicemapper/data
 Metadata file: /var/lib/docker/devicemapper/devicemapper/metadata
 Data Space Used: 2405.4 Mb
 Data Space Total: 102400.0 Mb
 Metadata Space Used: 2.3 Mb
 Metadata Space Total: 2048.0 Mb
Debug mode (server): true
Debug mode (client): false
Fds: 11
Goroutines: 11
LXC Version: 0.8.0
EventsListeners: 0
Kernel Version: 3.11.4-300.1.fc20.x86_64
```