File CRUD operations

* File create, delete, and get added.
* #get still needs a way to get the body and metadata at the same
   time. But also other response headers. May need a separate object for
   this–possibly a library-specific response object
* Not pleased with how Client#request is growing in complexity to handle
  various edges. More abstraction needed there.
The more I think about this the more I like `next_page` as it is. Avoiding customer surprise >> total consistency, and I'd definitely look for ways to enumerate the pages on the `PaginatedCollection`.