Add Router#namespace to match pattern with prefix

Hi,

I'm happily using chaplin.js in production(not yet released). Thanks for great product!

I added Router#namespace to DRY up the nested route definition. Our app has many long nested routes, so this method wil be very useful to clean up our `routes.coffee`.

example: 
```coffeescript
match 'users/:user_id', 'users#show'

namespace 'users/:user_id/', (match) ->
  match 'friends', 'users#friends' 
```