Pie Charts!

This branch includes Hassan's pie charts, including some additional bug fixes and features I added per request from @jaypoulz .

Commit Summary:
1.  Hassan's Pie Charts
2.  Added Dataset and Combined Datasets as Group By options
3.  Pie chart titles now update if the group by option or display field is updated
4.  Moved pie chart code to app/assets/visualizations/highVis
5. Changing the grouping field now works as intended
6.  Group selections are now properly updated in the chart
7.  Pie chart labels now reflect what the slice represents (In the case of grouping by dataset name, the dataset name is now displayed instead of "Slice")
8.  Slices of the pie chart now use the same color as the group they represent
     **Note, I could not test updating this with the color picker as it is temporarily broken, but I'm fairly certain it         will work.**

Towards issue #1495