Change schema to reflect usage of default

The schema defined default as indicating good imagery for the area, but this is not how the code treated it, which mapped it onto the JOSM XML attribute indicating to show it world-wide in the menu by default. This fixes the schema and updates some imagery
Update files

Forgot to run make after b1cedb5c4707fd73572677187f2a98f44ceeb808
Create file for Polish Geoportal (bug #68)
Run make for #73

Closes #73
Closes #68
Merge branch 'gh-pages' into default_description
Add another attribute to specify the "best" imagery for a region

Best is a fuzzy concept, and what to do with this is left up to editors.
Update README.md

P2 now supports editor-imagery-index
Merge pull request #72 from pnorman/default_description

Change schema to reflect usage of default
Set best for SA and Poland imagery
Merge pull request #76 from pnorman/bests

Set best for SA and Poland imagery