[korowai/testing] Finish comparators.

[ptomulik]

Interface:

• Korowai\Testing\Properties\ComparatorInterface,

Classes:

• Korowai\Testing\Properties\EqualityComparator,
• Korowai\Testing\Properties\IdentityComparator.

The should be extended with:

• identification method (like operator() or predicate()) which returns "equal to", or "identical to" respectivelly,
• detection that two instances provide exactly same predicate, or implement them as a singleton (in a container).

Also, write tests for them.

[ptomulik]

The first part is done. The "detection" is left to be implemented.