833 - Stacked Area Plots

Close #833.
Changing tests to check yCoordinates
Redid logic to use d3 stackLayout
Change tests to account for cross-browser inaccuracies
Merge branch 'master' into 833-stackedArea

Conflicts:
	plottable.d.ts
Adding quicktests
Quicktest doesn't choke if API doesn't exist on master
Restrict to vertical stacked area plots

Removed useless comment
Stacks on adding only.  Uses already stacked data in paint
Allowing for fill projection
StackedAreaPlots only take in QuantitativeScales

Code cleanup in general
Fixing class structure to extend off of stackedPlot
Hardened stack logic to stackedPlot.ts

stackedBarPlot.ts moved to child stackedPlot.ts
Removing unused instance var
Reverting stackedBarPlot.ts  To be changed in a different pr
Adding small test to check stacking
Hardened test for checking domain

Style fixes as well as some refactoring
Adding unused instance var
Updating stackedExtent in stackedPlot
Fixed extent logic