Add console.assert and console.debug (+ question about console methods)

(Initially via https://github.com/tmpvar/jsdom/issues/926, while porting over https://github.com/assaf/zombie/blob/master/src/zombie/console.coffee)

Unless I'm missing something, it doesn't look like any console methods other than `console.error` output anything in jsdom ([all point to this code path](https://github.com/tmpvar/jsdom/blob/master/lib/jsdom/level1/core.js#L868)). My questions are:

- If console output isn't currently exposed anywhere, would providing stubs for all console methods be sufficient?
- Should there be a way to capture console output for better testing? (and maybe also to expose externally?)

I'll add to this PR based on feedback.
Don't delete these (or change this test at all); we want to test that they *don't* put errors in the errors array. You could change the test description if it was confusing.