Error page improvements

// the follow code is write in modules:
// GR:=NewGroupRouters()
// GR.AddRouter("/login",&UserController,"get:Login")
// GR.AddRouter("/logout",&UserController,"get:Logout")
// GR.AddRouter("/register",&UserController,"get:Reg")
// the follow code is write in app:
// import "github.com/beego/modules/auth"
// AddRouterGroup("/admin", auth.GR)

add comments in logs package.

simplify condition test for trailing /

2. add hookfunc to support appstart hook

add comments for httplib package.

add comments for context package.

add comment in config package.

Added LayoutSections property to Controller