Add AirPrint and Mopria compatibility

[michaelrsweet]

We can officially support AirPrint and Mopria clients (including Windows 10) by reporting some attributes and DNS-SD TXT record keys. Should also add a way to report a list of ready media for iOS clients.

[michaelrsweet]

OK, here is the plan:

• Add proper "urf-supported" keyword attribute and URF DNS-SD key for AirPrint
• Add "document-format-preferred" mimeMediaType attribute with value "application/pdf" for AirPrint
• Add _universal sub-type for AirPrint
• Add "mopria-certified" text attribute and DNS-SD key with value of "1.3" for Mopria
• Add ReadyPaperSizes directive for cupsd.conf, defaulting to the DefaultPaperSize value plus 4x6, Legal, and Tabloid if the default is Letter and A5 and A3 if the default is A4.
• Add "media-ready" and "media-col-ready" with the set of ReadyPaperSizes that are supported by the printer.
• Add "image/urf" MIME media type to mime.types.

[michaelrsweet]

Development happening on "apmo" branch...

[michaelrsweet]

Related to PR #27.