Compile Condition: improve performance

- use dict['key'] instead of dict.get('key') which is much faster
Compile: build conditions elements as list
Compile Statement: return structure with check and body
Compile Check: group similar checks together
Compile Check: adhere @depend_property for @postprocess
Mapnik templates: update list of dependend properties

- add list for icon
- add image
- include final-casing-width in casing
Compile Check: don't include checks/postprocess for properties which are not used
Define @default_value
Compile Check: first eval default values, then postprocess
Mapnik templates: complete @depend_property list
Merge pull request #25 from plepe/performance

Performance improvements