Merge pull request #46 from alphagov/fix-node-evaluation

Apply node evaluation to resolve Interpolations

This will apply conditional logic, and value interpolations to node models, resolving them to content only representations before they're returned as part of API::State, eg

%{question} will be resolved to the answer to question 1

The rest of interpolation already worked, it just wasn't being applied.
Engine & Flow can take additional 'default' states