[GH-600] [PreUpdateEventArgs] Allows to add a new value on the entity changeset

This issue is created automatically through a Github pull request on behalf of egeloen:

Url: https://github.com/doctrine/doctrine2/pull/600

Message:

Hey!

I propose to add a `addNewValue` method on the `PreUpdateEventArgs`. This method allows to add a new value on the entity changeset when the entity changeset does not wrap the field.

In one of my app, I need to update a field if an entity is updated but the field is not updated at the beginning.