all: improve the method of import for internal packages

1. Change directory structure and Makefile to let GOPATH=`pwd`, so wherever the project directory is, just use make to build.

src/frp/cmd/frps/main.go

@@ -0,0 +1,27 @@
+package main
+
+import (
+	"os"
+
+	"frp/models/server"
+	"frp/utils/conn"
+	"frp/utils/log"
+)
+
+func main() {
+	err := server.LoadConf("./frps.ini")
+	if err != nil {
+		os.Exit(-1)
+	}
+
+	log.InitLog(server.LogWay, server.LogFile, server.LogLevel)
+
+	l, err := conn.Listen(server.BindAddr, server.BindPort)
+	if err != nil {
+		log.Error("Create listener error, %v", err)
+		os.Exit(-1)
+	}
+
+	log.Info("Start frps success")
+	ProcessControlConn(l)
+}