Add pre-publication validations.

This adds a new property to the models called ``is_publishable``,
which also gets set out in the response as ``isPublishable``.
This boolean value can be used to both stop a publication on the
backend and/or disable publication on the frontend.
Insert the is_publishable property into a binder's tree results.
Adjust the is_publishable property on binders and add are_contained_publishable.

This gives the cascading behavior a new property and allows the is_publishable
value put into a binder tree to be isolated to that specific model.
Adjust the are_contained_publishable to a trivalue on binders.

This indicates whether a binder is completly, partial or not publishable.
keep contained_publishable as Boolean
fix the tests for boolean
invert logic
Merge pull request #165 from Connexions/pre-pub-validations

Pre-publication validations