Removed orphan continent style.

Addresses #242.  Doesn't affect rendering because there's no continent
datasource layer.
Remove halos from highway link names

Part of #1 and #168. The halos came from the catchall at the end of
#roads-text-name.
remove support for unused tunnel=true and barely used tunnel=1
Remove airport symbols
Move polygon-clip into specific selectors

polygon-clip in the outer scope creates an unintended PolygonSymbolizer
with default gray fill. This causes the non-opaque, non-default amenity
attachment to blend with gray.
Update project.mml

use SQL equality operator
remove unused synthetic tag tunnel=no
Merge pull request #293 from mrwojo/fix-phantom-clip-poly

Fix dark place_of_worship at low zoom
Merge pull request #280 from mrwojo/remove-continent

Removed orphan continent style.
Merge pull request #281 from mrwojo/road-name-halos

Remove halos from highway link names
Merge pull request #291 from mrwojo/remove-airport

Remove unused aeroway=airport
Change the SQL subquery aliases to match layer names

This makes it easier when reviewing patches, since often the id appears
too far away from the query to make it into the diff. Also useful when
looking at e.g. pg_stat_activity
Move glaciers-text above non-text layers.
Make glacier text easier to read.

Increased text size, darker text and larger halo to help isolate the text from
backgrounds e.g. admin boundaries.
Make place names easier to read.

Larger, with larger halos. Slightly lighter black makes it less contrasty and
therefore a bit more pleasing.
Less intense colour for admin boundaries.
Per-layer opacity for admin lines, to reduce over-draw nasties.
Turn off line clipping for boundaries with dashes.
Bump opacity down, since there's fewer overdraws.
Lighter forests, and have symbolizer on a transparent background for easier customisation.