Add support for content staging scenarios

[gggeek]

This is similar to issue #57 , but it caters to a different usecase: content is edited on server A, and transferred to server B for display.

This is generic base functionality, and different customers might have different specific needs, such as fe.:

• stream all contents as soon a they are edited, or
• decide chunks of content to sync at a given time
• transform some data on-the-fly while syncing (eg. creator user, modification timestamp). We should focus on some specific scenarios and describe them in detail to come out with a definite set of requirements.

First requirements that come to my mind:

• allow 'upsert' migrations that would either create an object or update it if already existing (#245)
• check that the eZ kernel has enough Signal Slots defined to trap all content editing events
• allow the on-the-fly generation of migrations (maybe via a migration itself, of type migration/create)