Run hooks when contacts/identities are added/removed. #209

Now that the sync layer is separated from its clients, actions that used to occur within the DatabaseComponent, such as adding or removing private groups when adding or removing a contact, have to be handled by clients.

This patch adds a mechanism for clients to register hooks with the Contact/IdentityManager to be run when contacts/identities are being added/removed. The MessagingManager uses ContactManager hooks for adding and removing private groups, and the ContactManager uses IdentityManager hooks for removing contacts when an identity is removed.

I've also changed the way some components register with the LifecycleManager and EventBus.

This merge request depends on !64 (merged) - I'll merge them at the same time. Closes #209 (closed).