Ensure all after hooks are executed, even if one of them fail

[johanblumenberg]

Reopening this PR: #3281
See the details and discussion in the referenced PR.

[coveralls]

Coverage decreased (-0.04%) to 90.263% when pulling bd90921 on johanblumenberg:invoke-all-hooks into 04469bf on mochajs:master.

[johanblumenberg]

The first commit in this PR also fixes the issue pointed out by this comment:

mocha/lib/runner.js

Line 841 in 6535965

// XXX how about a less awful way to determine this?

[plroebuck]

We're not going back to assert -- use unexpected in your tests.
Won't recommend for review until merge conflicts are fixed, just so we're not wasting anyone's time.