Support executing CMS without installation, but from in directory different from its root.

Add a search path for isolate, which is built taking ../../isolate/isolate
with respect to the file that contains the module cms.grading.Sandbox.

Do the same also for the configuration file cms.conf.
Add a UniqueConstraint in Dataset on (id, task_id).

It's required, otherwise the ForeignKey fk_active_dataset_id
cannot be created.
Implement NullBackend as a /dev/null FileCacher backend.
Update cmsMake to task versioning and to use TaskTypes (fixes #131).
Never delete anything from a FileCacher.

The same file could be used by other structures present in the
database.
Fix some details in Luca's patch.

 * Add comments;
 * Use NullBackend for FileCacher.
FileCacher: add method to completely destroy the cache.
Destroy the cache after having finished testing.

It would be meaningful to save the cache for later testing, but
that would require some better thought architecture to make it
locatable by later executions of cmsMake.
Stupid PEP8 fixup.
Add a couple of Slovenian contests among testimonials.
Don't keep the sandbox by default.
Add small comment
Fix two bugs about insufficient and premature cleaning.
Making PEP8 happy.
Factor the retrieval of the psycopg2 connection in a dedicated method.
Documentation: use "database" instead of "cmsdb".

This alignes the documentation with the example configuration.
Use a better way to drop the whole database.

Unfortunately it requires one small step more when creating the
database. I don't think it is too invalidating, though...

Fix #153.
Fix a couple of my copyright years.
Remove an unused import.
Write the machinery required to make Importer and Reimporter fully modular.

Now there can be many loader, selectable at run time, with also
the possibility of autodetection.