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.
I wonder if it's worth making this `tap` unnecessary by dropping a `yield self if block_given?` inside CloudFiles::new.

Maybe not, test fixtures are a bit of a special case here.