new expression property
Merge branch 'master' of github.com:mapnik/mapnik-reference into expressions

Conflicts:
	latest/reference.json
convert older refs to new expression declaration
Merge pull request #79 from mapnik/expressions

Declare expressions for Mapnik 3.x
bump to v6.0.0 with new expression formatting
start minimal mocha tests
housekeeping
Add datasources ref dynamically if it exists
improve tests
few more tests
glob as devDep
fix version
strict linting for 3.x and above
Add warning to script
don't default to latest
makefile cleanup
python test: detect versions via glob
tests: detect versions via globbing
readme updates
move latest to 3.0.0