Allows modules to be imported from Node

Currently tracking.js modules can work in Node via sandboxing, see https://github.com/eduardolundgren/tracking.js/blob/master/test/utils/sandbox.js.

Maybe adding `module.exports` into each file would allow to require them from node without a sandboxing? Once it's done we can publish it to NPM.

Start with test/utils/sandbox.js and inspect how tracking.js modules are currently loaded under Node. Verify that the modules can be required without sandboxing and that the resulting package is suitable for publishing to NPM.

Written by the indexing model from the issue text.