Add functionality to check if device is connected properly.

Hey Renzo, Thank you for this very usefull library! I am using it for a current project, however I found that I am not able to see if the device is connected properly. As a solution to this, I would like to suggest that the return value of "_write.endtransmission()" is used to see if the I2C transmission was succesfull. For example, a "status" parameter could be added to the PCF8574 class, which indicates that the latest transmission has failed. It is also possible to return the result of "_write.endtransmission()" in the .begin() function, instead of using a void function. Feel free to do with this suggestion as you please. Best, Michiel