Extend the printer example to downloadable drivers.

This has nothing to do with PredicateInstaller, but makes the example a useful general-purpose printer installer.
Fix mistakes in previous commit
Fix minor security issue in printer example
Fix mistake in previous commit
Printer options for new printer
Printer example: patch defaults into the PPD
Seems like the previous change actually isn't necessary
Printer example now downloads PPDs from CUPS server
Merge pull request #8 from mkuron/printerdrivers

Printer example extended to new driver installation mechanisms