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.
http://devblog.avdi.org/2011/07/26/the-procedurefunction-block-convention-in-ruby/