quake ML validation using relaxNG schema

- validation now actually meaningful ;)
- two minor modifications from current official QuakeML 1.2 relax NG
- deactivating default stdout printing in validate routine, I would not
  expect a basic check routine that returns True/False to print messages
adapt two tests to be valid QuakeML
workaround for #559
changelog for quakeml validation
Merge pull request #560 from megies/quakeml_validation

Quakeml validation