New Connector accessing PostgreSQL/PostGIS tables

This connector connects to PostgreSQL/PostGIS tables, i.e. add access to plain tables but also to feature tables stored in a PostgreSQL database with PostGIS extension.

The connector currently is read-only, so changing and storing features is not possible at the moment. There is a test case available since https://github.com/52North/IlwisTests/commit/057703bed279536b4985eac1810f4f3284ddca09 including a demo. Note that for testing a database dump is necessary. Once the connector has write capabilities [it is planned to improve this](https://github.com/52North/IlwisTests/issues/3).