cmd/swarm, swarm/api/client: add HTTP API client and 'swarm ls' command (#3742)

This adds a swarm ls command which lists files and directories stored in a
manifest. Rather than listing all files, it uses "directory prefixes" in case there are a
lot of files in a manifest but you just want to traverse it.

This also includes some refactoring to the tests and the introduction of a
swarm/api/client package to make things easier to test.

swarm/swarm.go

@@ -195,7 +195,10 @@ func (self *Swarm) Start(net *p2p.Server) error {
 	// start swarm http proxy server
 	if self.config.Port != "" {
 		addr := ":" + self.config.Port
-		go httpapi.StartHttpServer(self.api, &httpapi.Server{Addr: addr, CorsString: self.corsString})
+		go httpapi.StartHttpServer(self.api, &httpapi.ServerConfig{
+			Addr:       addr,
+			CorsString: self.corsString,
+		})
 	}
 
 	log.Debug(fmt.Sprintf("Swarm http proxy started on port: %v", self.config.Port))