Add int setDeltaT and getDeltaT

I really love swRTC, but I was annoyed that I couldn't read back the DeltaT correction. Also I felt that using floats only for that was a disservice to the library, because an int with tenths of seconds would fits more easily in integer only Arduino sketches.

However, my first try using the forked library seems to indicate an increase of compiled size instead of a decrease. Maybe I did forget some class specific stuff for overloaded methods ?