Downloading the binaries through a proxy

[ulrichard]

Downloading the binaries through a proxy if either HTTPS_PROXY or HTTP_PROXY environment variable is set.

closes #29

[RCasatta]

Are HTTP_PROXY and HTTPS_PROXY a standard and do you have any reference of projects doing something similar (is cargo automatically using those)?

Is HTTP_PROXY only relative to http requests, thus we should need only HTTPS_PROXY?

If we decide to merge this we need to do also for bitcoind crate otherwise you will have the proxy issue anyway

[ulrichard]

It is more a widely used convention than a standard.
https://superuser.com/questions/944958/are-http-proxy-https-proxy-and-no-proxy-environment-variables-standard
In my experience, HTTP_PROXY is also used for https if HTTPS_PROXY is not set. Thus I implemented it that way. But it is also possible that this was just with the programs I used, and the majority behaves differently.
There is also the issue with the casing that is not really standardized. I have seen upper case being used more often.

Yes, I realized, that the same is needed also for bitcoind. I'm on it.

[RCasatta]

still requires rust-bitcoin/bitcoind#49