uWSGI and Isso start properly

- uWSGI runs Isso inside the container
- uWSGI and Isso can be configured using environment variables

The container has yet to be tested with an actual site.
Move uwsgi spool directory to data volume

Otherwise, messages in the queue could get lost if the container is stopped in
the meanwhile. There has also been an issue with the permissions on the
/tmp/isso/mail/ directory, which was not writable for uwsgi.
Check for mandatory environment variables on startup

Some environment variables are mandatory for the container to work properly.
Those variables are now checked on startup. If a variable is not set, a message
is logged accordingly and the container exits with an error code.

Closes #2.
Bump version
v0.2.0
Make the isso config distinguishable as a template

The isso config used in most places of the image/container is a template, until
the template variables are replaced with their actual values. I renamed the
file in the according stages, to make this fact more clear.
v0.2.1