Add new collections format

Initial stab at updating the Scala client to handle collections and fronts. Note, I haven't updated the README yet because neither fronts nor collections are available in v1.

There are also no added tests.

The content model has been complicated somewhat by the new collections format. We could make it simpler by overloading the `Content` type with optional fields and ignoring the difference between supporting and curated content. I've gone comprehensive at the expense of concision at this point, but it's definitely up for debate.