Clean up BarChart Functionality

While building a dashboard, I found myself needing the BarChart view. I cleaned it up and added a few features. Notable points:
1. BarCharts now support multiple series
2. Demo now has one BarChart

Let me know when you get a chance to take a look at this. I haven't worked with d3 in the past so some of my code may not be doing things the d3 way. One concern I have is using calculateX and calculateY that call the x() and y() functions set up by d3. I did this so that it could be extended in the future, for example adding a stacked bar chart would be easy with some simple changes to calculateX and calculateY.