Docker-based Processor

This PR allows launching any OMERO script (even non-official ones) within a docker container. The image I used for testing was:
 * https://index.docker.io/u/joshmoore/omero-process/ (image)
 * https://github.com/joshmoore/docker-omero/blob/master/omero-process/Dockerfile

To test, comment out the standard `ProcessorTemplate` from `etc/grid/default.xml` and uncomment the Docker-based one at the bottom. (There's a bug somewhere else which only allows one at a time. Deleting `dist/var` may also be necessary after the change.)

Regular scripts should continue to operate. But in addition, scripts uploaded by non-admin users should also be executable:

```
echo "import omero.scripts; omero.scripts.client('t.py'); print 'done'" >> t.py
bin/omero script upload t.py
bin/omero script list user
bin/omero script launch IDHERE
```

/cc @manics