Add Program.assertEqual

Summary:
This is so that users can write their own unit tests,
for use in the algorithms curriculum.

Test Plan:
- Put Program.assertEqual(2, 4); // see oh noes
- Put Program.assertEqual(4, 4); // see no oh noes

Reviewers: john

Subscribers: #cs

Differential Revision: http://phabricator.khanacademy.org/D12254