Merge pull request #122 from AndrewBuck/master

Update file contents comments and change low zoom hillshading.
Merge pull request #123 from AndrewBuck/master

Buildings now show up only at z14 and higher.
First shot of CONTRIBUTING.md
Change compare URL in README
Merge pull request #127 from AndrewBuck/master

Minor building tweak and the country and state changes we discussed on IRC.
Merge pull request #129 from AndrewBuck/master

Low zoom boundary fixes and one minor typo fix.
Use Legend page as preview link
Clean palette (fix #125)
barrier=border_control (fix #107)
Merge branch 'master' of https://github.com/AndrewBuck/HDM-CartoCSS into AndrewBuck-master
simpler management of cities and towns
Lighter state text color
Enlarge country text size
First shot in displaying cities at low zoom level

Thanks (again!) to @springmeyer for the shield-placements trick
Use shields for natural=peak
natural svg not saved...
Use `capital='yes' AND admin_level='2'` instead of `is_capital=country`

Thanks to @pnorman
Change syntax for tags WHERE clause
default to NULL for population
Add buffer-size (fix #140)