Coerce scale to a Number to make Mapnik happy

Without this, Mapnik emits `TypeError: optional arg 'scale' must be a number`.
Merge pull request #67 from mojodna/patch-1

Coerce scale to a Number to make Mapnik happy
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.