Remove hard-coded base host_parts.length of 2 and add support for site subdomains

I removed the hard-coded value of host_parts.length > 2 for has_subdomain?(), and I added support for git repos whose code is used for additional subdomains, e.g. example.com and api.example.com. I was also having problems getting Divergence to work behind a proxy (which is handling SSL), so now Rack::Proxy support a backend_uri option (https://github.com/ncr/rack-proxy/pull/15), which I added to Divergence in fix_environment!().