修改beego.go的NewGroupRouters FUNC中slice陷阱错误

这个pull request 已经更新为 #526 Update beego.go

修改beego.go的NewGroupRouters FUNC中slice陷阱错误
修改文件 beego.go
修改前文件位置代码：
```
line 31  return make([]groupRouter, 0)
line 36  func (gr GroupRouters) AddRouter(pattern string, c ControllerInterface, mappingMethod ...string) {
line 51  gr = append(gr, newRG)
line 54 func (gr GroupRouters) AddAuto(c ControllerInterface) {
line 60 gr = append(gr, newRG)
````
修改后 代码：
```
line 31  return make(GroupRouters, 0)
line 36  func (gr *GroupRouters) AddRouter(pattern string, c ControllerInterface, mappingMethod ...string) {
line 51  *gr = append(*gr, newRG)
line 54 func (gr *GroupRouters) AddAuto(c ControllerInterface) {
line 60 *gr = append(*gr, newRG)
```