Added the option to show only grid markings (without axes or labels).

Summary:
Deprecated props.showGraph in favor of props.markings, and defined
a way to deprecate widget properties going forward.

Test Plan:
Made sure that there were no errors on test.html or testrender.html when
setting "markings" to any of the supported values. Verified that "showGraph"
overrode "markings" when present as specified by the deprecation function.

Reviewers: jack, eater

Reviewed By: jack

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D4429