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.
Is there a more specific exception class you can assert here?

`docs_linkable` catches and re-raises the original `Exception`, too. Could make *that* more specific, to only catch transport-layer exceptions, and re-raise as members of our own exception hierarchy? The advantage being that callers could catch and handle Redrax problems independently of any other HTTP wrangling they happen to be doing at the same places on the call stack.