See also URL updates

Some project locations have changed.
Merge pull request #49 from pnorman/patch-1

See also URL updates
support latest mapnik 3.x (with custom variant)
travis: test only with Mapnik 3.x
v0.5.2
update changelog for v0.5.2
revert unintended change
fix compile of tests with mapnik 3.x
remove -Wshadow since its too noisy
rework testcase to fail if hashing mapnik::value is not working - refs mapnik/mapnik#2358
Merge branch 'master' of github.com:mapbox/mapnik-vector-tile into raster-clip
use unbuffered extent to subset raster