Make slug function visible outside of Revel package

Allows calling of slug function from controllers. Useful for redirecting with arguments (in conjunction with ReverseUrl)

----

I think it would be useful to be able to call the slug function outside of templates, for example to redirect.

```go
func (c Thread) SubmitPost(threadId int64, content string) revel.Result {

	// Validation and database interactions

	url, err := revel.ReverseUrl("Thread.View", id, revel.Slug(thread.Title))
	if err != nil {
		return c.RenderError(err)
	}

	return c.Redirect(url)
}
```