Support multiple .deb package source URLs

The official Debian packages are already broken up into "main", "contrib", and
"non-free" groups, which means that there is no single source of even
'official' packages. This patch adds support for using multiple package lists
in place of a single one.

Note: the order in which lists are specified DOES MATTER. If the same package
is available in multiple repositories, the repository listed first will take
precedence.
Make bin/deb2tcz.sh more robust

This fixes some variable name confusion in deb2tcz.sh and improves the handling
of more complicated packages (those with startup scripts).
Merge pull request #79 from alexkonradi/enhancement/multiple-deb-package-sources

Extend .deb package importing to support multiple sources