Adding support for extra headers to cf_client and cf_container.

Cloud Files are allowed to take additional headers, in particular Content-Disposition.  Rather than handle each individual header as a parameter, we can assign the headers as a dictionary that gets passed in.  This also allows for better futureproofing.

Details: http://docs.rackspace.com/files/api/v1/cf-devguide/content/Enabling_Browser_Bypass_with_the_Content-Disposition_Header-d1e2219.html

Scratching an itch here - I need to be able to display PDFs that have been uploaded inline, and that requires this header.  There currently is no way to set this header with Pyrax.