Starts erdom prototyping, adds document factories.

* Renamed erod_document_manager to erod_registry, it is shorter.
* Added document factory support to the registry to start and create
  documents not found in the registry.
* Changed internal representation of keys to a simple tuple.
* Added get_content request decoder and result encoder.
* Made erod_document_worker actually create a document.
* Added a random part to erod_documnt_verlog version number,
  if the document process die this number will change forcing
  a full refresh of the document.
* Started prototyping erdom using documents and dummy storage.