Added Duration class (#4)
Added IntervalInterface::duration() (#4)
Fixed bug in normalizer.
Added TimeSpanInterface and TimePointInterface::add/subtract() (#4)
Implemented TimePointInterface::differenceAsDuration() (fixes #4)
Updating travis config.
Updated readme [ci skip]
Update README.md
A few basic doco fixes (fixes #19) [ci skip]
Updated Calendar to use gmdate() for ISO/doomsday purposes.
Merge pull request #33 from koden-km/feature/gmdate

Updated Calendar to use gmdate() for ISO/doomsday purposes. (Fixes #23)