Move responsibility for testing elements from DataTester to TestingContext
Introduce ValueTester as an abstract parent to Int / Bytes test
Rename AbstractDataTester->AbstractTester and CompositeDataTester->CompositeTester
Rename various tester classes for consistency
Add test for Int.formatItem()
Add list() / ListTester
Extract interface from TestingContext and put it in se.fnord.katydid
Update README.md after stack trace changes
Bump version to 0.3.0-SNAPSHOT