ProxyFactory should return interface CheckWithProxy, not implementation.
big test refactoring, more unit, less it-tests.

- unit tests for FluentExpectedException
- enhancements in unit tests for other classes
- exclude IT tests from pitest, we want 100% from unit tests,
	besides IT, should not be mutated.
- preconditions: check if not null in some methods
- remove abstract parents of ThrowableAssertMock and ThrowableAssertMockRegister

Closes #4
Closes #3