Added groups

Allow the addition of related routes using groups.

```go
m.Group("/books", func(r Router) {
    r.Get("/:id", GetBooks)
    r.Post("/new", NewBook)
    r.Put("/update/:id", UpdateBook)
    r.Delete("/delete/:id", DeleteBook)
}, UserIsLoggedIn, PrepareBooks, ServeCoffee)
```
Instead of:

```go
m.Get("/books/:id", UserIsLoggedIn, PrepareBooks, ServeCoffee, GetBooks)
m.Post("/books/new", UserIsLoggedIn, PrepareBooks, ServeCoffee, GetBooks)
m.Put("/books/update/:id", UserIsLoggedIn, PrepareBooks, ServeCoffee, GetBooks)
m.Delete("/books/delete/:id", UserIsLoggedIn, PrepareBooks, ServeCoffee, GetBooks)
```