support for upstream proxy

Hi Michael,

being behind a corporate web proxy, I just noticed that ruby 1.9.3 (in contrast to ruby 2.x) does not use the HTTP_PROXY environment variable (see http://ruby-doc.org/stdlib-1.9.3/libdoc/net/http/rdoc/Net/HTTP.html#Proxy-method).

Therefore I added a further option --http-proxy which allows to define a proxy (host, port, user, password).

Best,
Thomas