feat(client): add proxy support

Added proxy support via environment variables.

closes #1567

[skip ci]
Generally I'd agree, but I was under the impression that `http_proxy` (lowercase) was more or less standard for proxies. See http://kaamka.blogspot.com/2009/06/httpproxy-environment-variable.html, http://www.putorius.net/2012/09/how-to-set-httpproxy-variable-in-linux.html, etc