Add a --disable-proxy option for disabling the proxy server.

A common problem people hit is that they end up going through a proxy
server when they don't want to. This change adds a --disable-proxy
option (ENABLE_PROXY in .reviewboardrc) that disables the HTTP(S) proxy
support.

Set an HTTP proxy (to a fake URL) and watched as post-review failed due to
not being able to find the proxy. Set the --disable-proxy flag and saw the
request go through.