BUG: Setting month greater than 9 fails

[GianlucaLocri]

Hi, first of all thanks for this library!
I think I found a small bug in function ds3231_set_time at line 217.
The result of bin2bcd() is masked with 0x0F and as a result tens are removed.

I fixed by changing
raw_time[5] = bin2bcd(timeptr->tm_mon + DS3231_MONTHS_OFFSET) & DS3231_MONTHS_MASK;
to
raw_time[5] = bin2bcd(timeptr->tm_mon + DS3231_MONTHS_OFFSET);