Add support for nested routes

Basically it wanted to use crossroads to write something like this.
```javascript
todosRoute = crossroads.addRoute('/todos', showStats)
todosRoute.addRoute('', showAllTasks)
todosRoute.addRoute('/{id}', showSingleTask)
```
This matches both `/todos` and `/todos/3`. In the first case it shows all tasks, and in the latter it shows only the task with id 3. Additionally it shows the stats in *both* cases.

Everything nested routes can do should be specified in *nested.spec.js*. I will also add documentation.