Add release dates to the changelog

Also use level 1 headers to comply with the conventions.

[ci skip]
Improve issue links in the changelog
Use new backports URL for wheezy

Debian Backport will be integrated into the main archive starting from
Wheezy:
  http://lists.debian.org/debian-devel-announce/2013/03/msg00007.html

Fixes: #5
Include apt recipe only after sources.list has been updated

Avoid extra `apt-get update` triggered by the apt::default recipe.
Update readme