In our environment we sometimes need the ability to post reviews through an HTTP proxy. This change adds support for an HTTP proxy that is specified either as HTTP_PROXY in the .reviewboardrc file or as --http-proxy on the commandline.

The change is bigger than it should be; it is this way to maintain backwards compatibility with the very flexible way we go about finding REVIEWBOARD_URL in any of many .reviewboardrc files. If we could assume that there is only ever one .reviewboardrc we could simplify the config loading.

This patch is based on a slightly older post-review version. If there is buy-in for this approach I'll apply this patch to a newer version of post-review and update.

post-review continues to work without specifying a proxy.
I have tested specifying a proxy both in .reviewboardrc as HTTP_PROXY and on the commandline using --http-proxy going through a plain old squid HTTP proxy.