Add support of composed durations

You can now build durations based on composed values, such as
'1 day 3 hours 6 minutes' for example.
Add tests for durations parsing and extraction
Update readme with composable durations examples
Fix Duration class examples
Merge branch 'feature/durations_composition' into devel
Version bump 0.2.0
Merge branch 'release/0.2.0'