Implements assert.null and assert.undefined

Here comes an attempt at getting `assert.null` and `assert.undefined` assertions (with their refute counterparts) as requested in #5. The assertions and expectations (expect form only) are working as expected, but specs like `.mustBeNull` or `.mustBeUndefined` are impossible, because `null` and `undefined` aren't objects.

Also note that the `.null` and `.undefined` notations aren't compatible with Internet Explorer (up to IE9) and other legacy JavaScript engines. You must use the form `assert['null']` and `assert['undefined']` instead if you need to support such legacy engines.