Have AreaPlot add a padding exception at y=0.
Instead of adding a constant exception at 0, use extent calculation to detect if there is a constant baseline.
If the baseline is constant, add an exception.
Add unit tests
Update the areaPlot so baseline exception changes if data changes.
Add additional unit test.
Change scale to expose _autoDomainIfAutomaticMode()
Also ensure automaticMode is re-enabled on autoDomain call
Refactor 'onDataSourceUpdate' into its own method
AreaPlot calls updateYDomainer on dataSourceUpdate.
AreaPlot.updateYDomainer calls _autDomainIfAutomaticMode
Refactor so AreaPlot descends from LinePlot rather than vice versa.

Having LinePlot descend from AreaPlot was nice and made sense since you can
see a LinePlot as a special-case AreaPlot in which there is no fill, only
stroke on the line. However, this broke down once we implemented logic to
keep track of a possible baseline at the bottom of the Area, and writing special
logic in LinePlot to turn the baseline back off seemed less elegant than just having
LinePlot not implement the baseline logic at all.
Merge branch 'master' into areaplot-exception

Conflicts:
	plottable.js
Merge branch 'master' into areaplot-exception

Conflicts:
	plottable.js
Merge pull request #602 from palantir/areaplot-exception

Have AreaPlot add a padding exception at y=0.