Add fractional ticks to the interactive number line

Test Plan:
 * Add an interactive number line
 * Try adding ticks for all three modes
 * Ignore the 1 1 1 1 1 1 bug
    * This has existed for a while
    * I will fix this everywhere in a separate commit

Reviewers: alex

Reviewed By: alex

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D5870