[MODEL] Added the ability to transform models during indexing

For instance, to add the `_parent` field to indexed documents:

    transform = lambda do |a|
      { index: {_id: a.id, _parent: a.author_id, data: a.__elasticsearch__.as_indexed_json} }
    end

    Article.import transform: transform

Closes #113
[MODEL] Minor changes and tweaks to the `transform` feature for import

Related: c4b42b5, #113