Official Quakeml based events

Switches all event related stuff to be based on official QuakeML, including validation via RelaxNG. Should work but not tested in its final stage yet. This means a major change and relies on other changes in seihub.core (e.g. xpath patch in https://github.com/megies/seishub.core/commit/58ec02589850e3db8c2a00fa5db9e12bc21fc1b1).

Do **NOT** merge yet. Do **NOT** merge unless barsch/seishub.core#8 is merged.