Adding support for proxy and protocol selection. 

Added three parameters to the cred map: proxyHost, proxyPort, and protocolStr. This allows someone using the clj-aws-s3 library to easily connect to a local Riak CS Vagrant cluster through:

    (def cred {:access-key accessKey,
               :secret-key secretKey,
               :proxyHost "localhost",
               :proxyPort 8080,
               :protocolStr "HTTP"
               })