Add support for `proxy_uri` and `use_ssl`

Most private object storage solutions support direct and proxy
configurations. For clients interacting with object stores in a direct
configuration, overriding `endpoint` works. In order to support object
stores in a proxy configuration, this commit adds a `proxy_uri`
option (along with a `use_ssl` flag for object stores without HTTPS
enabled).
Merge pull request #33 from hectcastro/hc-proxy-support

Add support for `proxy_uri` and `use_ssl`