Add ZMAP write support for earthquake catalogs

More formats are likely to follow later. Let me know if you need me to make any changes.

Best
Lukas
While technically correct, I think CSV is a more widely used and accepted term for all kinds of dialects (see e.g. [wikipedia](http://en.wikipedia.org/wiki/Comma-separated_values) or [python docs](https://docs.python.org/2/library/csv.html)).