Adding `Connections::remove()` to remove a connection.
Move up the itemization step to the model level && refactor tests.
Make the `lithium\data\Entity` & `lithium\data\Collection` handlers configurable.
Refactor `lithium\data\Model::_initialize()`.
Merge pull request #972 from jails/feature/override-entity-handlers

Allow to override handlers for `Entity` & `Collection`, close #958