Allow scrollbar to be horizontal.
Modify vertical layout to return left and right.
Update demo to use horizontal scrollbar.
Invalidate position when left-right motion occurs.
Add public method to get vertical state.
Update tests.
Change assignment to linter.
Fix lint errors.
Merge branch 'master' into horizontal_scrollbar

Conflicts:
	test/scroll_bar/ScrollBarSpec.js
Fix the selection bug.
Refactor entire class to eventually support peek and single.
Clean up initialization.
Clarify size assignments.
Fix bar size bug.
Remove unused import.
Refactor to remove use of layout and work on peek and single.
Add track offset to demo.
Add docs and remove personal comments.
Refactor test suite.
Rename track offset to something sane.