add training/label data for buildings

[andrewljohnson]

The analysis could be simultaneously analyzing images for buildings, along with the road analysis it already does.

1. download_labels.py needs to be extended to extract buildings, which isn't very hard
2. way_bitmap_for_naip needs to be extended to draw and then shade in buildings probably. This still isn't very hard, maybe a tad harder than step 1.

[bhack]

For building and urban areas see also benchmarks and state of the art in http://www2.isprs.org/commissions/comm3/wg4/results.html