Process Mapnik: process() returns (mapnik) text and columns

instead of just returning the mapnik text and using a global for storing the
list of database columns, return a dict with 'text' and 'columns'. A positive
side effect is, that only those database columns are added which are really
used.
Process Mapnik: instead of str.format use strtr(s, dict)

This enables to replace "[xxx]" too, but for now we use the same replacement
patterns as in previous versions
Process Mapnik: move generation of sql columns to separate function
Process Mapnik: build sql column: don't query default values

if a property only has the default value, don't add to query but replace by
value instead
Process Mapnik: build sql column: remove leading/trailing spaces
mapnik-3.0: remove wrap-character
Process Mapnik: only default value, fixed values as symbolizer parameters

If a property has only the default value, replace the respective symbolizer
parameters by fixed values; also remove them from the sql columns
Process Mapnik: build sql columns: bugfix, don't modify cache data
Merge pull request #49 from plepe/49-mapnik-performance

Performance improvements renderer