Add support for container labels

[mreithub]

Could look something like:

{
    "image": "foo",
    "labels": {
       "lbl1": "value1",
       "com.example.lbl2": "val2"
    }
}

[mreithub]

Note that labels are only supported since Docker v1.6 (API v1.17), so it's probably best to issue a warning if labels are used but not supported by Docker

[mreithub]

storing labels implemented in 6fb9d68 (reading them is still missing)