Read support for NEIC mchedr event file format.

Hi,
for a project of mine I wrote a parser for reading the [NEIC mchedr] event file format.

Code is here: https://github.com/claudiodsf/obspy.mchedr
It is written mimicking core/quakeml.py, but it has only read support (I guess nobody is crazy enough to save a catalog to this format!).

If it interests you, I can submit it as a pull request, though there are many things to fix (just look at `#TODO` and `#FIXME` in the code).

Let me know,
Claudio

[NEIC mchedr]: http://earthquake.usgs.gov/research/data/pde.php