Added Debian installer

The main changes are:

* The reintroduction of the `debian` template directory
* The reintroduction of the `debian` build target to the `Makefile`
* Adding the feature test to the `__init__.py` script (to satisfy the old plugin system)
* Adding an empty `__init__` to `suns_aa_motifs.py` (to satisfy the old plugin system)

`suns.zip` still works on the new plugin system, but now the Debian installer works for the old plugin system that comes on Debian.