Roundrobin

Support flashlight servers behind DNS round robin and similar schemes.

This depends on the changes in the roundrobin branch in getlantern/enproxy.

This patch involves changes to the command line.  The `-server` parameter becomes mandatory, and thus a `-role` parameter is added to specify whether flashlight is to run as a 'server' or 'client'.  I'm not super convinced about this particular interface, so I haven't updated the README yet.  I'll do so as soon as we decide on this. 
Another option would be to make `-server` optional for servers, and not setting the Host in the enproxy.Proxy if it's not provided in the command line.