Add 'select interval' problem exercises

Summary:
Add 4 exercises:
concave_up_and_concave_down_intervals.html
positive_and_negative_intervals.html
positive_and_negative_slope_intervals_1.html
positive_and_negative_slope_intervals_2.html

Test Plan: Tested locally

Reviewers: alpert, stephanie

Reviewed By: stephanie

Differential Revision: http://phabricator.khanacademy.org/D1