More features, tests

The mixer got somewhat rewritten to avoid copycat code. Now mixer reuses itself to traverse deep into objects.

Breaking changes:

• opts.clone does not exist anymore. It was confusing. You can clone/create objects yourself if necessary.
• opts.filter() second argument is not a key anymore, but targetValue. The key is the third now.

New features:

• opts.transform - allows to change any value which is being assigned to the target/destination object.

Many unit tests were added. Some bugs fixed.