Added proxy support

[3kt]

A proxy can now be used by using the -proxy parameter in both sblookup and sbserver. This overrides the default use of the environment variable HTTP_PROXY (or http_proxy). Please refer to https://golang.org/pkg/net/http/#RoundTripper for more info

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

:memo: Please visit https://cla.developers.google.com/ to sign.

Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks.

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• If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address. Check your existing CLA data and verify that your email is set on your git commits.
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[3kt]

The CLA should have been handled. I'm contributing as part of a company, and was told that all the required steps have been completed. Could you please confirm?

[colonelxc]

Code looks good. There is a bit of an issue in verifying the CLA.

• An email address associated with this github account needs to be in the corporate google group.
  • General information: https://cla.developers.google.com/about
  • Add that email to github: https://help.github.com/articles/adding-an-email-address-to-your-github-account/
  • See what CLAs you are a part of: https://cla.developers.google.com/clas
• The commits need to be associated with that same email address:
  • Doing this for every (new) commit: https://help.github.com/articles/setting-your-commit-email-address-in-git/
  • Amending existing commits: https://stackoverflow.com/questions/3042437/change-commit-author-at-one-specific-commit

Once those are taken care of, the bots will be happy, and I'll merge it.

[googlebot]

CLAs look good, thanks!

[colonelxc]

Thanks again for the PR!