namespaced mount

if i mount anmesia in a rails namespace, assets and links won't work because they're hard coded.
eg, css would be loaded from `/amnesia/css/...` instead of `/admin/amnesia/css/...`:

```ruby
namespace :admin do
  mount Amnesia::Application.new => "/amnesia", as: :amnesia
end
```

this patch makes link generation dynamic by using sinatra's [url helper](http://www.sinatrarb.com/intro.html#Generating%20URLs).