compare returns non-zero when images differ

exec will pass err whenever it sees non-zero result codes so there's no
point checking for it.
Upgrade node-mapnik to 1.1.2
Expose mapnik binding as MapnikSource.mapnik

This allows tilelive-mapnik dependents to register fonts, etc.
Merge pull request #69 from mojodna/expose-mapnik

Expose mapnik binding
Merge pull request #68 from mojodna/node-mapnik-1.1.2

Upgrade node-mapnik to 1.1.2
add travis
set buffer_size at render time (in two places for forward and back compatibility) - refs https://github.com/mapnik/node-mapnik/issues/175 and https://github.com/mapbox/tilelive-vector/issues/5
Revert "set buffer_size at render time (in two places for forward and back compatibility) - refs https://github.com/mapnik/node-mapnik/issues/175 and https://github.com/mapbox/tilelive-vector/issues/5"

This reverts commit ef89afc151af485cdd316a3d9eceb2f9e5711632.
add todo wrt to https://github.com/mapnik/node-mapnik/issues/175
bump to v0.6.0
fix engines declaration
bump to v0.6.1 with upgraded node-mapnik