GH-169: preFlush event and lifecycle callback

Pull-Request was automatically synchronized: https://github.com/doctrine/doctrine2/pull/169

For now, we have `@PrePersist`, `@PreUpdate` and `@PreRemove` callbacks. But it's not enough, cuz in some cases we need to run some entity method just before *every* `EntityManager#flush()` call (examples - translatable behavior and file uploading routines).

I've added and tested new `@PreFlush` event, which occurs during the start of the `EntityManager#flush()`, before any changeset gets calculated. This gives users ability to hook into flush process and prepare their entities to save even if they were not changed.