Add support for airprint & printer sharing to cups cookbook

This is the patchset ported from biola/chef-cookbooks#2 (Thanks to: `git format-patch` & `git am -p2`)


**Changes:**

 1. `cups::airprint`: Recipe to add AirPrint service definitions for sharing printers via Apple AirPrint
 2. Add `node['cups']['share_printers']` attribute (default: `true`) for configuring cupsd to share printers on local network
 3. Fixed broken `cupsd` service status detection (it was falling back to matching string `cups` in `ps -ef` output... which returned false positive: `cups-browsed`)