Take proxy environment variables into account

[xoolive]

Building fails when behind a proxy because the tarball cannot be downloaded. This easy fix lets ureq look at the http_proxy environment variable.

[Dirreke]

Great! Could you also add an instruction to the README.md file?

[xoolive]

I'm not sure what to say in the readme: "things work as expected if you set your http_proxy as expected" ?

[Dirreke]

Let's add a new Chapter named Proxy before Contribution and said that the openblas-src will try to detect proxy settings from the environment, i.e. HTTP_PROXY, when downloading the source of OpenBLAS.

[xoolive]

How about that?