Feature/Add taxonomy model to accommodate URI

[demery]

At present taxonomies is an array of strings on the project:

https://github.com/KislakCenter/VisualCollation/blob/development/viscoll-api/app/models/project.rb#L11

In the spec, VisColl schema a taxonomy can have a @ref:

https://github.com/KislakCenter/VisualCollation/blob/development/viscoll-api/public/viscoll-datamodel2.0.rng#L265-L273

A change needs to be made so that Taxonomies can have attributes.