Don't crash on bad command-line arguments

As amusing as it is to have servo --help die with SIGILL, it's not the best
user experience :)
Remove remnants of old reftest system

Back in 2012, Brian wrote a reftest system using Firefox via Selenium to
generate comparison images.  This seems like a pretty cool idea, but we
didn't maintain it and important pieces are now missing.

Let's remove it from the working tree to avoid confusion with the current
reftest system, and I'll open a ticket about revisiting this idea.

One pair of old reftest files seems usable as a new reftest and that move is
included in this commit.
auto merge of #1767 : kmcallister/servo/misc, r=metajack
layout: Rewrite the float context.

This rewrites the float context to avoid dynamic failures resulting from
`.clone()` misuse. It also renames the float context to the simpler
`Floats`. The new version is modeled on WebKit's `FloatingObjects`.
Add check for non-Text children for document.title.(fixes #1742)
auto merge of #1771 : lpy/servo/issue1742, r=Ms2ger

see #1742
Remove no-op implementations from Element and add some spec links.
auto merge of #1774 : Ms2ger/servo/element-noops, r=jdm
layout: Stop racing on the JSManaged borrow flags
auto merge of #1772 : pcwalton/servo/borrow-flags-race, r=jdm

r? @jdm
auto merge of #1770 : pcwalton/servo/rewrite-float-context, r=kmcallister

This rewrites the float context to avoid dynamic failures resulting from
`.clone()` misuse. It also renames the float context to the simpler
`Floats`. The new version is modeled on WebKit's `FloatingObjects`.

r? @kmcallister
Renamed HTMLIframeElementTypeId to HTMLIFrameElementTypeId
Add url getter to Page and fix users of Page url with it.(fixes #1762)
auto merge of #1776 : lpy/servo/issue1762, r=jdm

see #1762
Move Error, Fallible, ErrorResult out of utils.rs (fixes #1749)
auto merge of #1773 : sawrubh/servo/issue1749, r=Ms2ger
Implement "resize" DOM Event.

* We cannot automate this feature.
* So this testcase is a manual test.
auto merge of #1783 : saneyuki/servo/resize, r=jdm

#1715
auto merge of #1781 : hgentry/servo/mozilla-servo, r=metajack

Simple change. I hope I'm using github correctly; I'm not too familiar with the PR system. Fixes #1777.
De-@mut the script crate.