Added "Taxonomy" dropdown

The query builder should allow to choose which taxonomy to search for.
Ideally that will affect the fields available to the user.
Updating the QB

Selecting a different taxonomy should cause the query builder to update
its fields. It's hard to see because we have only one taxonomy right now
and we don't have a query that gets the proper fields yet; this is just
the connection between Angular and Rails.
Ignoring stuff...
Small HTML refactor

Ensured that Bootstrap is included, and started to build a more solid
HTML/CSS structure. Hopefully it's the first step towards a prototype
that's not entirely hideous.