Example plugin

This still needs tests and documentation, but generally works. 

This connector exposes schemas defined in the `metadata-uri` catalog property.  The `metadata-uri` can reference a local file or a remote http resource.  The table data files are simple CSV (no escaping supported) fetched from a location relative to the `metadata-uri`.  The sample `etc/catalog/example.properties` file has URIs for local file data and a sample remote dataset hosted on Amazon s3.