Add srs autodetection to node-srs for sqlite files

[willwhite]

No description provided.

[willwhite]

I was working on this in millstone. It's easy to query for the SRID when the user has provided a basic table name, but it's going to be very difficult to determine the geometry table and SRID if the user has entered a complex SQL statement.

Deferring for now.

[springmeyer]

I think ideally mapnik datasources could provide a function to call like ds.keys() or something which would allow you to gather all the internal data that the plugin has figured out (specific to that datasource). So, in this case, after the ds has been created you could ask it for its geometry table name and its key field, etc to be able to get all the right info to then query for the srs in the geometry_columns table.