Merge pull request #29 from scrapinghub/issue28

Adds missing return methods fixing issue #28
Allows to specify a docker image form chronos

A hack that allows to pass a serialised json instead of a plain
command. The serialised command contains the docker image name and the
actual command to be run. This allows us to specify the docker image
from the Chronos framework. More to come…
- Removes exact-match flag to fix provisioning