FIX: issue #3 (date switching from Sun to Mon)

Problem was in days calculation for DS1307:

firmware thought that 0 is Sunday and 6 is Saturday, but
in fact, Sunday was 7 (when 6 incremented). Monday worked
well, but index 7 in timetable gave data not from required
part of RAM, so there were a mess in the memory.

Signed-off-by: Maslov Nikita <wkernelteam@gmail.com>