Database reload

**This is a pull-request against the lua branch**

This PR proposes a database-reload. It changes the documentation on the use of
osm2pgsql, and adds a .lua file for preprocessing.

This database import aims to be backwards compatible with older versions of
the style.

This resolves (at least part of) #1504.

 #Hstore
Adding the hstore option to osm2pgsql allows the database to use all keys.

This PR proposes using hstore for new keys, and using traditional columns for
old keys. This allows us to stay compatible with old versions of the style.

According to [@pnorman's benchmarks](http://www.paulnorman.ca/blog/2014/03/osm2pgsql-and-hstore/),
using hstore without dropping columns would lead to a 9% size increase and a
0.38% speed decrease. Given the benefits of having all columns available, I
would consider this acceptable.

 # Make polygon/linestring decision based on value
This is based on the following unmerged PR to osm2pgsql:
osm2pgsql-dev/osm2pgsql#346

It makes the polygon/linestring decision based on the tag value, in addition to
the tag key. In particular, highway=services and junction=yes are now treated as
polygon, while leisure=track, man_made=embankment, man_made=breakwater,
man_made=groyne, natural=cliff, natural=tree_row, historic=citywalls,
waterway=derelict_canal, waterway=ditch, waterway=drain, waterway=river,
waterway=stream, waterway=wadi, waterway=weir, power=line, and power=minor_line
are now treated as linestring.

This resolves #1362, resolves #137, resolves #268, and resolves #892.

 # Rendering order
The new .lua file creates a osmcarto_z_order value in the database, which stores
the rendering order we use. This will allow us to simplify the road queries.

 # Recommend -G flag for multipolygons
This resolves #59.

 # Import ele on buildings
This resolves #101.

 # Delete more meta-tags from imports
The .lua file drops some more meta-information from imports that is of no
need for rendering.

This is based on the following unmerged PR to osm2pgsql:
* osm2pgsql-dev/osm2pgsql#532

INSTALL.md

@@ -5,7 +5,7 @@ You need OpenStreetMap data loaded into a PostGIS database (see below for [depen
 It's probably easiest to grab an PBF of OSM data from [Mapzen](https://mapzen.com/metro-extracts/) or [geofabrik](http://download.geofabrik.de/). Once you've set up your PostGIS database, import with osm2pgsql:
 
 ```
-osm2pgsql -d gis ~/path/to/data.osm.pbf --style openstreetmap-carto.style
+osm2pgsql -G --hstore -d gis ~/path/to/data.osm.pbf --style openstreetmap-carto.style --tag-transform-script ~/osm/openstreetmap-carto.lua
 ```
 
 You can find a more detailed guide to setting up a database and loading data with osm2pgsql at [switch2osm.org](http://switch2osm.org/loading-osm-data/).