Adds support for get_content and get_children requests.

* Fixed a lot of bugs.
* Uniformize get/find functions; if and item does not exist,
  get will create one if possible when find will just fail.
* Changed document get_content and get_children result to be
  records; a new result 'unchanged' is returned when FromVer
  is the same a the current version.
* Changed the 'interest' functions to 'watcher' functions.
* Added support for get_content and get_children in the
  websocket connection.
* Added serializer module for erdom documents.