Rename mkimage-debian.sh to mkimage-debootstrap.sh to more clearly reflect its true function
Refactor mkimage-debootstrap.sh to be much more full-featured

This includes the following:
- options to control certain parameters
- an option specifically for creating tarballs directly, esp. for dockerbrew
- the addition of the updates and security repositories for Debian images
- the addition of the universe, updates, and security repositories for Ubuntu images
- more correct tagging of Debian images
- tagging of Ubuntu image versions (12.04, 12.10, etc) and latest tag for LTS
Merge pull request #2222 from tianon/debootstrap

Renamed and refactored mkimage-debian to mkimage-debootstrap to more clearly reflect its true function