workflow support

[dbu]

#117 made the controller more generic to allow handling custom workflows. we should provide some custom workflows in this bundle as examples. an obvious one would be publish workflow, to set the published flag / start & end time. this would be an interesting one, as it also depends on the document type whether this workflow should be enabled. if we then can also limit to some privileged users, we have an interesting use case :-)

the idea would be to tag workflows in the di container and register them with RestService. we need some way to also support access checks for workflows, maybe as a tag option that registers a special access checker (do we need security voters in the access checker?).
we also need a way to automatically to include the javascript widgets for each workflow in create.js

[dbu]

note that there is one example of a custom workflow, for handling delete https://github.com/symfony-cmf/CreateBundle/blob/master/Workflow/DoctrinePhpcrDeleteWorkflow.php

it is defined as a service, but loaded explicitly when the configuration option is set. this mechanism is not extensible.