Add CommonJS / NPM support

[felixfbecker]

This adds CommonJS support to this module by detecting the presence of module.exports, and if it exists, exporting the factory function instead of calling it on the global Highcharts object. This is in line with how the Highcharts exporting module is used in CommonJS.

It also adds a package.json, if this PR gets accepted please publish this module on NPM.

• I prefixed the name with highcharts- since it is a plugin
• I added highcharts as peerDependency since that is how version constraints for plugins should be defined. The homepage mentions at least 3.0 is required, so I set the version constraint between 3.0 and the next major version.
• I took the version from the manifest.json (1.3.8), even though the homepage actually says 1.3.6
• I took the description from the homepage (first sentence)

This allows one to require('highcharts-export-csv')(Highcharts) in a CommonJS environment like Browerify. Global use is still supported.

[TorsteinHonsi]

Thanks!

[felixfbecker]

Awesome. And don't forget to npm publish :)