remove unused highway=gate from style
remove deprecated and unused man_made=power_wind
Replace road shields with SVGs

Fixes #693
Fixes #680
Fixes #203
Fixes #202 (mostly)
Fixes #146 (mostly)

Cross-reference #294 mapnik/mapnik#2020

Starts organization for #508

Replace existing PNG road shields with new SVG ones. This allows for easier adjustment of colour and width as well as sharper high-resolution rendering.

At the same time, a number of other shield related bugs are fixed.

To aid in organization, a new shields directory is created.
stop rendering [tourism = bed_and_breakfast]
Merge pull request #694 from pnorman/svg_shield

Replace road shields with SVGs
Merge pull request #695 from mkoniecz/bb

stop rendering [tourism = bed_and_breakfast]
Merge pull request #692 from mkoniecz/mmpw

remove deprecated and unused man_made=power_wind
Merge pull request #691 from mkoniecz/hg

remove unused highway=gate from style
Remove amenity=bus_stop - closes #667

Obviously keeps highway=bus_stop as before.