Fixes function lookup_from/2 result format.
Fixes a bug and do some renaming.
Changes erod_indices and erod_document_page interface.

* Uniformize erod_indices interface so it could use a list instead
  of a tree if needed.
* Changes the page interface a bit, use single get_content/2 function
  that returns the page or a patch if available.
Adds a gb_sets wrapper.
Adds the first version of the document's view data structure.
Adds the first version of the document data structure.