Download snapshot from proxy

[bombless]

I cannot access Amazon S3 from my network, so I'd appreciate if the build script can fetch snapshot through proxy.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @alexcrichton (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. The way Github handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see CONTRIBUTING.md for more information.

[alexcrichton]

Could this actually revert parts of aceba88 instead? I think at this point we've had enough problems with urllib2 that we may just want to switch back to using the curl command line.

[bombless]

@alexcrichton Weird, I can download without proxy when using curl(I checkouted that https://github.com/rust-lang/cargo/blob/079d90235d861ea98a396808b6b7097e129ce564/src/etc/dl-snapshot.py and make, the downloading process along with the build ran fine).

I'd be happy if we just revert to the version before that commit, as I don't enjoy coding in Python :)

[alexcrichton]

Yeah reverting back to the original version should also be fine (I haven't changed this script in ages)