Guys, I have solved the problem!
I'm a little embarassed about what was the real cause of the problem, but I think posting here can help other people with the same problem.
When I saw the pin out responsible for I2C in my arduino board, I visited a forum and the guy told me that SDA whas A4 and SCL was A5. BUT, I have Arduino LEONARDO, and the I2C pin out for this board is digital 2 (SDA) and digital 3(SCL). For this reason I was having problems reading and writing on the RTC. I discovered this thing not intentionally, reading Atmel's datasheet. I want to apologize for this mistake and thank by the time that everyone spent on this!
A special thank for Pete and Nick Gammon, that really helped!