Add support for proxy
Unneeeded check as githubWebUri is guaranteed non-null at this point.

Personally, I'm fine with having a defensive check like this, but @skottler didn't seem to like this in a review comment in https://github.com/jenkinsci/github-oauth-plugin/pull/15/files so I'm respecting his opinion by removing this check.
Merge pull request #15

Conflicts:
	src/main/java/org/jenkinsci/plugins/GithubSecurityRealm.java