fix days-in-year calculation bug.

The algorithm idea was accurate but the lookup table was wrong.
It's also a bit simpler to just include the 0th element since
the conditionals to test for it likely occupy more than one
byte anyway.  Tested this version with multiple months in and out
of leap-years and it's producing correct results now.

Signed-off-by: Chris Evich <chris-arduino@anonomail.me>
Rtc_Pcf8563 specific version of leapYearsBetween

In order to calculate a duration which could span
any number of leap and non-leap years, it's necessary
to account for them.  This method returns the number
of leap-days for years from start to end (exclusive).
The century-bit handling behavior matches that of
the chip and other calculation methods here.

Signed-off-by: Chris Evich <chris-arduino@anonomail.me>
Merge pull request #11 from Yewess/utilities

fix days-in-year calculation bug.
Merge pull request #12 from Yewess/days_between

Leap Days between