Switch "suburbs" with "neighbourhoods", reported by adamkaml1.

http://forum.openstreetmap.org/viewtopic.php?pid=454387#p454387
Missing seocndary shields on z12, improve placement.

An all zoom levels use minimum-distance to control distances as the
merging of fragmented ways in our SQL query doesn't seem to work well
enough.  Set minimum-distance to 100 everywhere (150 for secondaries),
and spacing to 400px.
Highway shields also on z10 and z16.
Use minimum-padding on shields to avoid partial shields.

Also slightly better sorting.
Add line-spacing's for better multi-line shields.  Clear cache.

Note minimum-distance is deprecated in favor of "margin" in some mapnik
version (not 2.2, something later).