Rename app to events
Support different actions per Action type

Allow to have different functions called per event.
Factor out the function to import an object
Refactor the available actions

We want to allow more actions than simply "run".

Also, merge the ConditionalAction to the base one; it can run by default
and always return True.
Add {edge,level}-triggered actions

The first one supports an "edge-triggered" event. That is, it provides
support for the kind of actions that take place only on status changes.

The second one is just for completness.
Make the base class usable by itself

Keep things simple. One should be able to use the Action class as base
class and everything should work.