Implemented comparison methods on: Date, DateTime, Duration, TimeOfDay,
TimeZone, Interval. (Fixes #34)
Added missing use statements.
Moved differenceAsSeconds() into AbstractTimePoint class.
Merge pull request #38 from koden-km/feature/comparison-methods

Implemented comparison methods (Fixes #34)
Moved Duration and TimeSpan interface into TimeSpan namespace, removed unnecessary AbstractDuration and DurationInterface.