Adding abandoned buildings at z18.

![abandoned buildings](https://f.cloud.github.com/assets/430157/1230395/8cc84622-27d6-11e3-9b35-f9971149c4ae.png)
I drawing ways with abandoned:building=* on them like normal buildings, but with an X over them.  They show up at z18.

The rendering style works as shown in the screenshot, but it will not function properly until the DB that the rendering is done from is re-imported using the new default.style file for osm2pgsql that pnorman committed with [this commit](https://github.com/pnorman/osm2pgsql/commit/51b81f481669d90d885e5f24eb0e517f83b4b781).