Restore aeroway line rendering

This reolves #927.
Merge pull request #928 from math1985/aeroway-lines

Restore aeroway line rendering
Use a specific version of carto for travis

This works around mapbox/carto#368, but it's more to lock down versions
and keep CI as a test solely of the stylesheet, not of any components.
Add a preview image.
Update some of the text to reflect current status.
Add the maintainers.
Move installation instructions to a separate file, since the README has become unweildy.
Add a description of the versioning system.

I didn't know how this would work when we first started, but it's
settled down now so worth writing it out.
I always like seeing alternatives when I view other projects.
Add missing periods
More Markdownification
Explain about the shapefiles a bit more
Merge pull request #933 from pnorman/docs-grammar

Docs grammar
Merge pull request #932 from pnorman/travis_carto_mapnik

Use a specific version of carto for travis