API design for flexible time intervals.

Close #1212.
Add jsdosc.
Merge branch 'develop' into time_axis
Wrap steps into an array.
Introduce steps.
Add jsdoc, remove implementation.
Fix naming.
Introduce tiers.
Change names.
Add test.
Expose intervals.
Merge branch 'develop' into time_axis

Conflicts:
	plottable.d.ts
	plottable.js
	src/components/axes/timeAxis.ts
	src/utils/utils.ts
Add tests.
Adjust scope of PR.
steps -> step.
Formatters.
Add doc.
Merge branch 'develop' into time_axis
Fix merge.
Hide marks.