Add in Interaction.BarHover

This interaction takes in a BarPlot and calls callbacks when the user
mouses over a bar or mouses out of a bar.

Close #761.
Remove commented-out line, compact classed() call.
Merge branch 'master' into 761-hover-interaction

Conflicts:
	plottable.d.ts
Unhover callback is now passed last-hovered bar.
Merge branch 'master' into 761-hover-interaction

Conflicts:
	plottable.d.ts
	plottable.js
Clarify some Doc, compact a function declaration.
Add a default hover style (non-hovered bars fade out).

Remvoed some extraneous hover css.
Have unHover callback be triggered whenever the user leaves a bar

It used to not trigger if they immediately entered another bar.
Merge pull request #809 from palantir/761-hover-interaction

Add in Interaction.BarHover