Create new multi-line shields

This expands the shield generation script to cover multi-line shields,
and commits those shields. It doesn't yet use them.
Split concurrent refs into a multi-line shield

This new SQL takes the refs for a way, splits them on ;, and rejoins
them with newlines. When doing this it also calculates the number of lines
and the length of the longest line.

Fixes #670 as best as possible pre-Mapnik 3.
Also fixes #701
Merge branch 'master' into multiline_split_refs
Merge branch 'multiline_split_refs' of https://github.com/pnorman/openstreetmap-carto into pnorman-multiline_split_refs

Conflicts:
	project.mml
Merge branch 'pnorman-multiline_split_refs'