Delete all themes except "Brick" for major refactor
Great refactor for mapnik 2.3, most significant changes are:
1. Increase contrast a bit for better look on cheap LCD screens
2. Layer based buffer seize
3. Waterbody rendering now supports seamless inner shadow
4. Semi transparent land cover
5. Roads now uses grouped rendering, don't require old 4x outline->casing->inline->arrow data sent from database
6. Use shield symbolizer for place names and poi names

Note 5 and 6 are still in progress
Update roads data; Use group-by in mapnik instead grouping in database
Move resources to mapnik/res
- Replaced placename, poiname
- Road adjustment, added pedestrian
Make link casing thinner
- Patching async postgis parameter
- Compile @2x styles as well as original
- Added a new green color for sports
- Added a tunnel layer for google a-like tunnel render effect
Add back old "duotone" theme and renamed it to "newspaper"
- Separate different theme shield resources
- Add "nebula" theme back
- Each theme has its own set of shield markers
refactored data processing sql
Consider highway/railway when deciding road type, also sync road label type with road.
Make us shield background grey instead of warm white
Update theme with new applied brick sql
- All views are in brick schema
- Generations are in "natural" order all->gen0->gen1
Update README, minor config adj
Improved road shield rendering
Change general shield resources to imagemagick generated PNG
Use functions instead of duplicated sqls
- Change default Metatile size to 8x8 (16x16 costs too much memory)
- Added "wisteria" back and renamed it to "shell"