Allow for marker param on RackSpace CloudFiles

issue #147

CloudFiles api limits getFiles to 10k files, so you need a marker param to be able to designate a file offset.

I overrode the download parameter to allow for an object of query parameters.  I haven't looked through the rest of pkgcloud so I'm not sure if there's a better standard to follow or if any of the other storage APIs require similar functionality.  I'm open to other implementations if something else makes sense.