JSON plugin write support for Catalog

Here's a minimal version for plugging in the json dump routine with Catalog directly. Still misses documentation and tests..

 I don't know if this is useful or what the normal use case for the json routines looks like, but here is how you would plug it into Catalog, [like @krischer mentioned](https://github.com/obspy/obspy/pull/670#issuecomment-27380665).