More robust event hooks naming system. Closes #80.

To emphasize the fact that they are tied to a method, all
``on_<method>`` hooks now have ``<method>`` in uppercase.

- ``on_getting`` renamed to ``on_fetch``.
- ``on_getting_item`` renamed to ``on_fetch_item``.
- ``on_getting_item_<item_title>`` renamed to
  ``on_fetch_item_<item_title>``.
- ``on_posting`` renamed to ``on_insert``.