mediawiki::hhvm: add 'furl' cli tool

Adds /usr/local/bin/furl, a cURL-like command-line tool for firing off FastCGI
requests. A follow-up patch will use this script to automate warm-up requests
for MediaWiki on HHVM.

Change-Id: I77e8f98330efd3758e9a78c79e0a4be4ab59a9e7