Add option to restore helpers

Sometimes users want to restore helpers. This pull request adds this option.

The `render_dismissible_helper` method now takes an options hash. 
Adding `restorable: true` to the options renders the content, an close and an open string.

``` haml
= render_dismissible_helper 'sample', restorable: true do
  %h1 Hello World!
```

The JavaScript takes care of hiding, displaying everything and of course sending the notifications to the server.

Old style rendering works as expected - restorable is false per default:

``` haml
= render_dismissible_helper 'sample' do
  %h1 Hello World! - Hide me once and you'll never ever see me again
```

The only thing missing is tests. Those will be added soon. 

What do you thing? Would you include this upstream if tests are green? :)

Thanks for the awesome gem!