Reading and ReadingGroup clone behaviour

Previous behavior

Previously a new configuration could use the same ReadingGroup and Readings already existent, sharing them with others. This also apply when cloning configurations.

The problems with that

If the owner of a Reading destroys it, the previously created ranges would still reference a Reading that doesn't exists anymore producing errors.

Our solution

So, to deal with that we choose to always recreate a new object.

Enhancement

Keep track of which object is clone of which and them propagate possible changes according to the user preferences.

See: https://github.com/mezuro/mezuro/issues/28