Allow rendering collections inline in templates

Experimental implementation of #58.

The idea would be that if you have a view `groups/show`:

```html
<h1>{{name}}</h1>
<h2>Members:</h2>
<ul>
{{*members}}
  <li><a href="{{url}}">{{name}}</a></li>
{{/members}}
</ul>
```

... and a presenter:

```ruby
class Groups::ShowPresenter < Curly::Presenter
  presents :group

  def name; @group.name; end
  def members; @group.members; end
end
```

Then we'll try to wrap each "member" object in the presenter `Groups::ShowPresenter::MemberPresenter`. If the presenter doesn't exist in that namespace, it'll go down one level, i.e. `Groups::MemberPresenter`, and lastly it'll look at just `MemberPresenter`. This is how `const_get` works. That way, you can decide at which level to implement the presenter.

The presenter will look like this:

```ruby
class Groups::MemberPresenter < Curly::Presenter
  presents :member

  def name; @member.name; end
  def url; user_path(@member); end
end
```