3.0 range

Resolves https://github.com/cakephp/cakephp/issues/3304

I decided to go with the solution formed at the end of the discussion - "switch range() to inclusive for 3.0".
This is probably better than to add another validation rule to overcome the deficiencies of the current implementation.

One can use a custom (own) validation rule rangeExclusive() for exclusive ranges from there on if needed.
But that sounds like a really rare use case to me.

I will update the migration guide / docs afterwards.