include helptext for http-proxy

[yvesf]

Its helpful to include a howto for setting http_proxy environment variable, example (this commit depends on socks-related changes and is therefore not compatible to milanw/springer_download@master): yvesf/springer_download@75e4a086a0a16aa072b7

the python behavior regarding http_proxy is described in http://docs.python.org/library/urllib.html#urllib.urlopen