Add support for PostGIS as a layer source

[jerluc]

PostGIS is a pretty common data storage option working with geo data, so it would make sense to support this backend.

At a high level, these are the pieces I'm hoping to implement:

• [x] PostGISConfig
  • [x] Database connection parameters (i.e. DSN)
  • [x] Database relation spec (i.e. schema + table name)
  • [x] Default source fields or column expression mappings
  • [x] Geometry field or column expression mapping
  • [x] Accept an optional custom query in place of the database relation spec (e.g. to support "view" functionality)
• [x] PostGISSource
  • [x] Translate tile request into a SQL query
  • [x] Construct a feature collection from the resulting SQL rows
  • [x] Accept s query parameters to specify additional source fields / column expressions
  • [x] Accept q query parameter to specify additional source filtering / WHERE expressions
  • [x] Run runtime check on boot that determines if the user is read-only

[jerluc]

FYI, I've started work on this in https://github.com/StationA/tilenol/tree/postgis