Added Keen.Dataviz class.
Added Keen.Dataviz class.

Created new Keen.Dataviz class and using it inside the Keen.Visualization constructor. This allows us to keep the same API that but allow much greater accessibility under the hood.

Glathrop/keen-js,andrehjr/keen-js,Acidburn0zzz/keen-js,vebinua/keen-js,vebinua/keen-js,ipartola/keen-js,jbpionnier/keen-js,chrismoulton/keen-js,chrismoulton/keen-js,amplii/keen-js,ruleant/keen-js,lawitschka/keen-js,amplii/keen-js,ipartola/keen-js,ruleant/keen-js,allanbrault/keen-js,jbpionnier/keen-js,andrehjr/keen-js,lawitschka/keen-js,Glathrop/keen-js,allanbrault/keen-js,Acidburn0zzz/keen-js