New parser, options for username and printer.

A new parser makes order of parameter irrelevant and makes it easy to add new options. This added some complexity to the script, but it looks worse than it is ;)
Two options added: specify username with `-u user` and printer with `-p sw*` (monochrome) or `-p farb1` (color).
README.md was updated accordingly.

Some output allows user to review the files to be printed and eventual mistakes in the given parameters before doing anything.

Tested using debug output and dead trees. Please review ;)