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
Printer example now injects a passthrough filter.

This is necessary to change the MIME type to application/postscript. If we don't do this, the remote CUPS server does not run through pstops again and does not count the pages printed.
Merge pull request #9 from mkuron/printerdrivers

Printer example now injects a passthrough filter.
Printer example: process job on server