fork.js, start implementation of assynchronous fetch() to replace get() on large sets:
  - fetch( receiver ): Fetches the content of the source set, possibly in chunks.

      Parameter:
        receiver: function that will be called for each chunk of data from
          the source.

          Each chunk is delivered as a array of objects.

          After the last chunk, the receiver is called one last time with an
          empty chunk: [].

  - transform(): transforms an array of values into an other array of values
      according to the current pipelet role.

      Every pipelet should either implement transform() if it is a stateless,
      or fetch() if it is statefull.

  - use transform() in add(), remove(), and update()

  - Set.fetch( receiver ): sends set content to receiver function