Previously, 'python demo.py' would simply install and start the
'classic' tracker, using the anydb backend.

As we are working on more templates (notably now one for software
development), it would be good to provide more fine-grained control over
feature selection. As a first step, I have added options to select the
template, as well as the backend. This requires a little hack, as the
'minimal' tracker doesn't have what the demo.py script assumed.

In the long run, I think it would be best to put the tracker-specific
setup closer to the tracker templates. Therefor, we need to add hooks
that individual tracker templates could bind their own configuration
procedure to, such that 'python demo.py' would become a multi-step
process. The first step is the selection of the template, the second the
(template-specific) configuration of the latter.

(Example: the new 'devel' tracker template will allow bindings for
revision control, so the user may want to select among CVS, Subversion,
etc.)

Obviously, the simple 'python demo.py' command should continue working;
all configuration is optional and shouldn't destroy the simplicity of
setting up a (default) demo.

How does this sound ?