[data] Refactor and document

[backflip]

• Refactor, i.e. use Array.forEach over _.each
• Document every function with JSDoc annotations and in README
• Improve performance and structure (consider https://github.com/unic/estatico/pulls?q=is%3Apr+is%3Aopen+label%3A%22data+layer%22, /cc @marbor3)