build: add node.js module support

[robertkowalski]

This patch enables Node.js module authors to define ccurl as one of their dependencies in their package.json. It's then build for the current platform and Node version on installation time of the module.

Main functions of ccurl are exposed in index.js.

Usage:

npm install ccurl # given you published the module to npm

const ccurl = require('ccurl')
ccurl.ccurl_pow(/* ... */)

Notes:

I'm happy about feedback. I believe this would make development for iotaledger/wallet and other Node.js based projects easier.

Another option is to use a separate repository (in or outside the ioto org)

However in this place we have already travis hooks etc that could be used for automatic publishes to the registry when the C code changes. It also made things easier with path management for the build-result.

Remaining tasks:

• [ ] publish to npm