Update gdaltest.sh to use gdal_translate.

The new gdal no longer includes gdal2xyz, so the gdaltest.sh script
is updated to use gdal_translate -of xyz instead.
Add test re: rasterizing polygons on cell centers.

Fixes #620.  This commit adds a test that confirms that the example
included in #620 is being rasterized correctly -- or at least that
the behavior matches gdal's.
Merge pull request #629 from joshmarcus/issue_620_rasterizer_test

Add test re: rasterizing polygons with vertices aligned with cell centers.