Example code for async function

[springmeyer]

Most users of this repo are going to be building node C++ modules because they want to leverage the C++ threadpool (https://2015.foss4g-na.org/session/geodata-processing-nodejs) for writing async functions.

We should extend the HelloWorld class to expose a simple async function.

Some references:

• Most sophisticated, fully Nan way to do this: Refactors Bindinds to use Async Workers Project-OSRM/node-osrm#193
• Simplest example from node-mapnik (which does not use Nan, but more direct libuv): https://github.com/mapnik/node-mapnik/blob/a91e322e60a7e9911a69f39e025570993def6960/src/mapnik_image_view.cpp#L437-L551

[mapsam]

Working over in #3

[springmeyer]

closing, will track at #3

[springmeyer]

Actually, going to re-open, digest Project-OSRM/node-osrm#193 (comment), and then will share next actions on improvements to #3 here, when I have them.

[springmeyer]

Tracking at #5 works, closing.