Workflow content update

Updates the workflow response to be the same as what brian posted in #212. Adds versioning to the workflow model and so closes #206.
I think this is pretty cool, looking at the internals the strings array and the placeholders should never get out of sync but it could be problematic if it did. 

I've extracted the collector to the sub class that collects, https://github.com/camallen/Panoptes/commits/edpaget-add-versioning-to-project-content. Check it out i can submit a PR if you think it's not required by default in the base visitor class.