Add a handful of date-related utility methods

* isLeapYear()
* daysInMonth()
* daysInYear()
* whatWeekday()

I made them all pure so they can be used with stored values.  For
example, calculating durations across month/year boundariess in
arbitrary terms (i.e. days, minutes, seconds, etc).  This also
helps provide enough information to fill out the standard time_t
structure.

Signed-off-by: Chris Evich <chris-arduino@anonomail.me>
Merge pull request #10 from Yewess/utilities

Add a handful of date-related utility methods
Great job! Thank you.
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>