Add privet to connector

[jacobmarble]

Add privet capability for local discovery and printing.

https://developers.google.com/cloud-print/docs/privet

[jacobmarble]

Privet should support offline printing for users with special privacy concerns. Offline printing means that a print job never leaves the physical site:

1. job originates on the user's workstation or mobile device
2. workstation passes job to the connector
3. connector passes job to CUPS with processing requirements (some are normally handled by Cloud Print in Google's data centers)
4. CUPS sends job to the printer

[jacobmarble]

Quick clarification. #35 will add local discovery/printing as an option. Before this issue is closed, another pull request will make cloud discovery/printing optional, so that either cloud, or local or both can be used to share printers. This is important for low-bandwidth circumstances like #40 and for privacy-sensitive content that should have no chance of leaving the local subnet.

[mceachen]

I rebuilt my connector, but the new connector can't read previously-built configuration files:
F0821 17:33:57.088850 5796 connector.go:82] Printer attributes missing from config file: document-format-supported goroutine 1 [running]:

I re-created the config (which meant the printers needed to be re-shared), but when I changed local_printing_enable to true and restarted connector, the printer was then visible from a chromebook on the same LAN (yay), but after clicking "print" in the dialog, an error popped up and nothing printed (boo).

What debug logging would be useful?

[jacobmarble]

I need to document this better.

After an upgrade, it's useful to run $ connector-util -update-config-file to add new config options. Our internal deployment automates this.