use a local version of mason instead of writing into ~/.mason

fixes #526
add to $PATH instead of using aliases
Merge branch 'local-mason'

Conflicts:
	.gitmodules
	styles
Remove unused
line-offset -> line-gap-width
More concise
Axe the 16-px line width limit
v6: min-zoom -> minzoom, max-zoom -> maxzoom
Reinstate 16px line width limit
Implement v6 filters
Use v6 styles
Update styles
Render ⇢ layout, style ⇢ paint

https://github.com/mapbox/mapbox-gl-style-spec/issues/166