Quakeml validation

Based on RelaxNG schema now. One tiny modification of current official schema was necessary (see http://intensity.usc.edu/pipermail/quakeml/2013-April/000146.html). Some tests had to be adapted to be valid QuakeML. Also includes a workaround for #559.

In principle `isQuakeML()` could be based on `validate()` now but this would mean some sloppy files not getting read without explicitly stating `format="QuakeML"`.

Leaving the XSD files in the repo for now.