Support for manipulation of outbound documents via callback functions.

The ``on_getting``, ``on_getting_<resource>`` and ``on_getting_item`` event
hooks are raised when documents have just been read from the database and are
about to be sent to the client. Registered callback functions can eventually
manipulate the documents as needed.

Please be aware that ``last_modified`` and ``etag`` headers will always be
consistent with the state of the documents on the database (they  won't be
updated to reflect changes eventually applied by the callback functions).

Closes #65.