add content_encoding argument to upload_file and store_object

Added to both the client and container versions of these functions.
This is to add support for compressing objects before uploading
them to cloudfiles.

See http://docs.rackspace.com/files/api/v1/cf-devguide/content/Enabling_File_Compression_with_the_Content-Encoding_Header-d1e2198.html
Merge pull request #67 from joeshaw/store-object-headers

add content_encoding argument to upload_file and store_object