Change from null checks to coercive truthiness checks on non-primitive parts of the public API.
Rationale: This will be a little more idiomatic for javascript consumers without changing semantics, since
an object (ie. any non primitive) type will always return true if it is defined.

This would enable usage like:
`new Plottable.Component.Gridlines(xGridlinesEnabled && xScale, yGridlinesEnabled && yScale)`
[tweak] oh, semicolons....
Merge branch 'develop' into falsy-not-null-check

Conflicts:
	plottable.js
	src/components/plots/areaPlot.ts
	src/components/plots/plot.ts
Merge branch 'develop' into falsy-not-null-check

Conflicts:
	plottable.js
Merge pull request #983 from palantir/falsy-not-null-check

Change from null checks to coercive truthiness checks on non-primitive p...