Allow specifying a http proxy in the settings file

This change adds two new settings, proxy_address and proxy_port, to the
settings.rb file. If they are present, then all requests will go through the
specified http proxy. If they aren't present, then no proxy is used. The
http_proxy environment variable is still respected if no proxy is specified in
the config file.
Merge pull request #16 from mivok/proxy_support

Allow specifying a http proxy in the settings file