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,
           :proxy-map {
             :proxy-host "localhost",
             :proxy-port 8080,
             :username this-user-name,
             :password this-password},
           :protocol :http
           })

This pull request supersedes my last pull request, #44.