Squashed commit of the following:

Refactor buildings code

Replace the old buildings SQL and MSS. This involves resulting changes to landcover stylings to handle landcover which was previously in buildings.mss.

Stops rendering supermarkets in a crazy pink to fix #520. Superceeds #550.

Moves the rendering of train station areas to landcover. Fixes #327. Fixes #389

Removes outline differences based on a distinction that no one fully understands. Superceeds #533. Fixes #68

Rebased in 6b2a4de6ae2c7c685debb465ca424542c6d4fbfb by math1985 <github@matthijsmelissen.nl>
Correct SQL error

Rewrite of e7fe196600f9d05785a16ec09b241d48ce55d284
Add missing bracket
Merge remote-tracking branch 'upstream/master' into buildings_rewrite

This adds the landcover labelling, which is significant
Conflicts:
	landcover.mss
	project.mml
	project.yaml
stop attempts to display major buildings on z10 z11, special display across all levels
even the largest building in the world by footprint would be barely visible on z10 z11 so old building_major layer was not useful
now major buildings (place_of_worship for start) are displayed specially across all zoom levels with visible buildings
separate layer is necessary to ensure that building line of major building will be rendered on top of building line of touching building
note that major buildings will be rendered on top of not major buildings, including major buildings with lower layer tag
note that now aeroway terminal without building tag now will not be displayed
Merge remote-tracking branch 'pnorman/buildings_rewrite' into buildings_rewrite_new

Conflicts:
	project.mml
	project.yaml
Merge remote-tracking branch 'mkoniecz/buildings_rewrite' into buildings_rewrite_new
Merge branch 'master' into buildings_rewrite_new
Add WHERE clause to buildings-major layer

This should improve performance.
Merge branch 'master' into buildings_rewrite_new
Make buildings more gray

Colour proposed by vholten in https://github.com/gravitystorm/openstreetmap-carto/pull/565#issuecomment-51318931
Make buildings a bit darker on low zoom levels

On low zoom levels, buildings have no outline, so they need to be a bit
darker in order to have the same visual effect.
Merge branch 'buildings_rewrite_new' of https://github.com/math1985/openstreetmap-carto into math1985-buildings_rewrite_new
Merge branch 'math1985-buildings_rewrite_new'