Report more versions of dependencies in node-mapnik versions object

[springmeyer]

trafficstars

We currently report node, v8, boost, mapnik, and cairo. Let's add more. Critical ones include:

• libpng
• harfbuzz
• freetype

[mapsam]

Would also be nice to make sure these versions are reflected in the README #depends section

[springmeyer]

Thinking more about this: probably the best way to accomplish would be Mapnik core collecting the versions of all deps (since they are mapnik core deps node-mapnik) and then making them available via mapnik-config like mapnik-config --harfbuzz-version.

[tjwebb]

harfbuzz

mapnik lists this as an optional dependency. Is it not optional?

[lightmare]

harfbuzz is not optional. Only some of its later features are.