Add Hover ability to Plot.Scatter.

Currently uses a modified "Closest Point" algorithm:
   - If the cursor is over any circles, returns the closest of those circles.
   - Otherwise, returns the closest circle within some upper bound.

Progress on #848.
Modify _getClosestStruckPoint to combine return statements.

Ternary expressions.
Add quicktest for Interaction.Hover on Plot.Scatter
Add missing semicolon.
Merge pull request #1230 from palantir/hover-scatter

Add Hover ability to Plot.Scatter.