Allow nonunique x/y values on BarRenderer/HorizontalBarRenderer

Previously data binding was keyed on the x-value, so if bars shared
the same x-value only the first would be drawn (The same thing for
shared y-values on HorizontalBarRenderer).

Close #435.
Update built files
Add tests to ensure one bar per data point.
Merge branch 'master' into 435-bar-multiple-values

Conflicts:
	plottable.js
Update built files
Merge pull request #444 from palantir/435-bar-multiple-values

Allow nonunique x/y values on BarRenderer/HorizontalBarRenderer