Fixed validation for MONTH rule.
Fixed test for COUNTRY rule.
Changed assertion.
Fixed typos in comments.
Small changes in source code.
Changed Utils to Util.
Some changes in Callback's business logic.

Now CustomerReturnCallback can process only processing transactions.
Now PaynetEasyCallback can process only new transactions.
Callback fields definition moved from AbstractCallback to concrete callbacks.
Fixed comments.
Small changes in source code and tests.