gamification ready

[nekromoff]

Add config rules to make gamification possible.

Let's start with this one:

• bringing bicycle to an empty stand (as opposed to a one with bicycles) - e.g. add free X credit

But allow others to be added later (maybe as hooks or callbacks?) with separate controller for this.

[ewooonk]

As an additional idea: make a bike free to use if it not has been used for X days. For example, if it is placed on a remote location.