Core: Cache window.clearTimeout in case it gets mocked

window.setTimeout is already cached; doing the same for clearTimeout
may reduce race conditions causing TestSwarm to sometimes stop executing
tests in the middle.

Closes gh-511
Ref jquery/testswarm#298
Ref jquery/jquery/#1499
CSS: Updating qunit.css for consistency

Summary:
Use uppercase hex colors for consistency and 0 before point.
Use the short hex consistently.
Use the numbered font-weight properties.
Remove the vendor prefixed properties.

Closes gh-508
Closes gh-512
Assert: Extend throws to handle errors as strings

Before this patch, assert.throws accepted only a regexp to check for string
typed thrown errors, otherwise it would replace the expected to null and
consider its value as the assertion message.

Ref gh-506
Assert: Extend throws to accept Error instances

Fixes gh-476
Closes gh-506
Core: Extend QUnit.config.urlConfig to support select-one dropdowns

To make it easier for projects like jQuery UI to provide a dropdown that
let's the user select the jQuery Core version to test against. Can
display any key/value pairs (or just keys). Selection will be available
via `location.search` and parsed as `QUnit.config`.

Fixes gh-385
Closes gh-443
Addons: Remove last remnants

No immediate replacement available, yet. Feel free to take the deleted
code and host it in your own GitHub repo, we'll link to it on
http://qunitjs.com/plugins

Otherwise, use plugins like grunt-contrib-qunit to run QUnit tests in
PhantomJS.

Fixes gh-420
Ref gh-470