Add GitHub URL in main file

So upstream can be easily found.
Fetch test case dynamically and mention upstream in readme
travis: Enable on hhvm

Fixes #1
readme: Clarify that this is project is a port

Closes #4
Add !important support for four-value rules

The regex change was given by Bartosz Dziewoński at
https://bugzilla.wikimedia.org/show_bug.cgi?id=61440#c1 .