Rendering casing: Bugfix calculating final casing-width

- older Mapnik versions don't accept calculations in parameters, therefore we calculate the final casing-width internally. We also need to calculate possible final casing-widths in preprocess.pl
Default Base-Style, Mapnik 2.0: Remove 'offset' and 'casing-offset'
Objects: show duration of map query (uncomment line)
Eval::centroid: new function
Eval: Replace function by CASE..WHEN construct
Parse Condition: Bugfix reading values in quotes
Eval: Bugfixes comparison, convert NULL to ''
Compile Statement: Bugfix unset tags
Compile: optionally add profiling information
Compile Conditions: a condition might compile to an empty equation
Compile Condition: return null instead of 'true'
Compile Condition: where-match ignores assigned tags (prefixed by '.')
Compile Where: remove TODO which was completed in 31ef40a
Compile Where: check zoom levels of statements with assignments and their use
Eval: line_interpolate_point: catch errors
Eval: azimuth(), line_interpolate_point(): check parameters