Add https proxy and basic proxy auth support

This addresses issue #11 and issue #13.

Things changed:

* implements CONNECT for https proxy
* implements Basic authorization for proxies
* adds https_proxy and http_proxy attributes (tied to corresponding ENV vars)
* demotes the proxy attribute to a generic fallback

The last change is an incompatible one, but I think the overall consistency of proxy attributes is worthwhile.

Not yet implemented:

* Testing of different attribute and ENV var cases

I'd like to get reactions and feedback before merging to master.