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

Now it includes all sorts of goodies, including a bunch of command line arguments, very much improved Ubuntu support, and adding some nice repositories to `/etc/apt/sources.list` by default (especially security-related repositories).

Here's a list of a few of the things:
- options to control certain script parameters
- an option specifically for creating tarballs directly, esp. for dockerbrew (cc @shykes @shin-)
- 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

/cc @jpetazzo (because you've shown interest in this script in the past)

This is also in reference to #2208, #824, #2042.