Fix #72 and fix #74

[rpiaggio]

Fix:
#72: On FieldGroup.commit() fail, provide access to underlying Exception
#74: Integrate Vaadin's automatic DateField<->java.sql.Date Converter

[henrikerola]

Thanks a lot for your contribution!

[rpiaggio]

You are more than welcome and thank you actually for this project!

Please be advised that the change #72 actually breaks compatibility in cases where the string from CommitFailed was accessed (either via error, which could be remedied by adding a @deprecated def error = exception.getMessage) or via pattern matching (which I don't think can be remedied). These cases should be quite rare anyway.