Merge branch 'release/0.2' into develop
Added ChangeLog entry for last release.
Allow new sources to be added.
Allow extra sources to be added to `/etc/apt/sources.list.d`.
Describe how to add apt components to the sources.list.
Bumped version number.
Merge branch 'feature/sources' into develop
Images that already existed when the script is run will now have `apt-get dist-upgrade` run on them.

Also made some updates to the documentation.
Specify the directory to use when entering the schroot

We now specify a directory within the `schroot` to use when running
commands to avoid problems with running `mkschroot` from a location that
doesn't exist within the chroot.
Updated the PyPi classifiers
Use specified variant in debootstrap.
Put variant into setup in configuration file
Fix the HTTP proxy configuration for debootstrap
Added more information about variants

Also changed the layout around.
Handle variants for debootstrap and the configuration files

The variant can now be specified and alters the way that `debootstrap`
is run and the way that the `schroot` is entered.
Merge branch 'feature/variant' into develop
Bumped version number
Merge branch 'release/0.3'