Additional navigation proposal

In a couple of my projects, I used rails_admin as my application framework. It covers 80% of what I needed and I only had to implement a few home made controllers to handle the 20% left.

Note that to accomplish this I used the following technique: https://gist.github.com/1086055 which allows to reuse the awesome Activo theme of Rails Admin for my whole application.

Now, I needed to insert some additional links to my home made controllers into the Rails Admin navigation.

Another use case would be if I wanted to have a permanent link always in the navigation: for instance, let's say I have a customer model, and I want to always see a link to "Add new customer" in my navigation (from every admin pages). I could use the feature as well.

This pull request is a first stab at it, which I think is not ideal:
1) I don't like the configuration syntax that much
2) It's missing tests and docs (to be fixed of course)

The syntax currently looks like this

``` ruby
RailsAdmin.config do |config|
    config.nav_before_models = {
        'foo' => {
            'label' => 'Foo',
            'url' => 'foo/foo',
        },
        'bar' => {
            'label' => 'Bar',
            'url' => 'bar/bar'
        }
    }
    config.nav_after_models = {
        'stuff' => {
            'label' => 'More stuff',
            'url' => 'more/stuff'
        }
    }
end
```

Ideally, I wanted to use the RailsAdmin::Config::Navigation class, but it's being deprecated. The syntax could have looked like this:

``` ruby
RailsAdmin.config do |config|
    config.navigation.before_models do
        link :foo do
            label 'Foo'
            url 'foo/foo'
        end
        link :bar do
            label 'Bar'
            url 'bar/bar'
        end
    end
    config.navigation.after_models do end
        link :stuff do
            label 'More stuff'
            url 'more/stuff'
        end
    end
end
```


What do you think? Any suggestion on what would be the best approach?