expose constructor for simpler coffeescript / es6 extension

Allows plugins to be written in coffeescript (or es6 classes) natively:

Example (could be used with an extraction):

    module.exports = class Histogram extends Keen.BaseVisualization
      initialize: -> @render()
      render: ->
        @_chart = new google.visualization.Histogram(@el)
        google.visualization.events.addListener(@_chart, 'error', @handleErrors)
        @update()
      update: ->
        data = google.visualization.arrayToDataTable(@data.table)
        options = Keen.utils.extend(@chartOptions, {
          title: @title or ''
          height: parseInt(@height)
          width: parseInt(@width)
          colors: @colors
        })
        @_chart.draw(data, options)
      handleErrors: (stack) =>
        message = stack['message'] || "An error occurred"
        @trigger('error', message)

    Keen.Visualization.register('google', {
      'histogram'   : Histogram
    })