Added `Presenter` class

Presenters are simple models that can be use to store data and logic
related to a view. For instance, a presenter might store the state of an
expandable view and can be used as in `stickit`:

  class SomeView extends BaseView

    events: 'toggle'

    toggle: ->
      @presenter.set
        expanded: ! @presenter.get 'expanded'

    presenterBindings:
      ':el':
        observe: 'expanded'
        update: ($el, val) -> $el.toggleClass 'expanded', val

    initialize: ->
      @presenter = new Presenter
        expanded: false

      super
Merge pull request #6 from trabian/feature/presenters

Added `Presenter` class