Added createFromDate() static methods to Year/Month interval classes.
(Fixes #37)
Changed createFromDate() to createFromTimePoint() as suggested. (Fixes
Merge pull request #39 from koden-km/feature/easy-create-interval

Added createFromDate() static methods to Year/Month interval classes.
Expanded TimeSpanInterface considerably, reimplemented Date[Time]::add/subtract() in terms of TimeSpan (#32).
Completed period implementation (#32)
Updated typhoon checks.
Merge branch 'feature/period' into develop
Updated links in README.md [ci skip] (fixes #41).