fix proxy handling

In previous implementation the proxy handling was buggy and incomplete.
This patch add the following:

- with HTTP connections, we connect to the proxy and pass the URL as
  absolute folowing the RFC
- with HTTPS connections we are using CONNECT and the connection is then
  upgraded to SSL.
- add a {connect, ProxyHost, ProxyPort} option to the proxy setting so
  you can force hackney to use connect to a proxy.
- pass the Proxy-Authorization header when the proxy need
  authentication.

fix #76