Support merging databases

[Riebart]

It may be necessary to merge databases, which is simple since the events are all given unique IDs. If the IDs are not unique between the two DBS, and the contents of the two records with the same DB, then... something will have to give. But merging should usually be a simple process.

[Riebart]

Now that hooks for git synchronization are in, being able to resolve merge conflicts with a custom DB merge logic would be handy.