Simplify framework initialization

This PR aims at a complete implementation of the feature set requested by https://github.com/lotus/model/issues/33

- [x] Instantiate global mapper through `Lotus::Model.configure`
- [ ] Make Lotus::Model to register adapters instances, and to specify a default one.
- [ ] Because of the previous requirement, consider to introduce a configuration and duplication mechanisms, like we do with Lotus::Controller and Lotus::View.
- [ ] Mapper should allow to specify adapters for a collection.
- [ ] Provide a single entry point to make the setup automatic.
What if we use [`Hash#default`](http://www.ruby-doc.org/core-2.1.2/Hash.html#method-i-default-3D) here? 

```ruby
adapters = {}
adapters.default = 'sql'
adapters[:redis] = 'redis'

adapters[:redis] # => 'redis'
adapters[nil] # => 'sql'
```

But this is just an idea.. ;)