Add hover interactivity to Plot.Line and Plot.Area.
Refactors for clarity; calculate closest point during search.
Support unsorted data by switching to linear scan for closest point.

Going forward, probably will have an option to set a different algorithm.
Semicolon...
Add unit test for _getClosestWithinRange.
Add test for Plot.Line._doHover(); rename fakeHoverTarget --> hoverTarget.

It's a real target...
Attempting to diagnose Travis failure...
Further Travis investigation.
Further Travis experiments...
Put experiment before failing test...
Switch to using attr() check instead of getBBox()
Can't into closestPoint :(
Merge branch 'develop' into hover-line-area
Small refactors, Unit test covers more cases.
Merge pull request #1295 from palantir/hover-line-area

Add hover interactivity to Plot.Line and Plot.Area.