Added WITH_LIBEV/WITHOUT_EXTERNAL_POLL options.
Added C definitions for LWS_USE_LIBEV and LWS_NO_EXTERNAL_POLL
Added LWS_NO_EXTERNAL_POLL modes.
Started adding LWS_USE_LIBEV modes.
Added watchers to wsi objects, init them at creation, start/stop on POLLIN/POLLOUT.
Don't start watcher if context isn't inited.
Basic libev stuff is in place (though not totally working).