Move apt class parameters to a ::params class.

This brings no behaviour change yet, but will allow to set class parameters'
default value depending on system facts.
Use the main Debian archive's URL as the default URL for backports on Wheezy.

The backports are now fully integrated with the main archive.

See:
https://labs.riseup.net/code/issues/4270
https://lists.debian.org/debian-devel-announce/2013/03/msg00007.html