Added opportunity to register mocks.

Allows to register mock for specified class in unit tests:

- (void)test_MyClass_foo_method_is_invoked
{
    id myClassMock = [OCMockObject mockForClass:[MyClass class]];
    [[myClassMock expect] foo];
    [self.injector registerMock:myClassMock forClassOrProtocol:[MyClass class]];
    
    [self.testObject doSomeLogic];
    
    [myClassMock verify];
}