Introduce source schema

As described in #5 database schema should be used. Instead of having a source database, we put all sources in one database schema called sources.

Currently we have google buildings and microsoft buildings. Our OpenStreetMap data is imported with the tool imposm. imposm uses three different schemas for import, production and backup and handles all tables inside them. Thats why we still need a separate schema for OpenStreetMap data and can't include this data in the sources schema.