Add an HTML file where JavaScript changes text color

I wanted this for testing incremental layout, but it's also useful to
illustrate a bug I just found.
auto merge of #618 : kmcallister/servo/test-html, r=jdm,jdm

I wanted this for testing incremental layout, but it's also useful to illustrate a bug I just found.

(PR attempt #2, hopefully Bors won't flip out this time)