[WIP] API cleanup

This PR is a major cleanup and aims at making the library practically usable:

 - [x] Move event triggering from `ChangeSet` to `UnitOfWork`
 - [x] Rename `ChangeSet` to `ChangeMap` (naming is still amibiguous
 - [ ] ~~Move `Change` to correct namespace~~
 - [ ] ~~Simulate actual rollbacks along with commits - see #9~~
 - [ ] ~~Redefine events to be triggered (eventually move to an embedded event manager) - see #10~~
 - [ ] ~~Create a small bootstrapper or setup API to ease usage - see #11~~
 - [ ] ~~Handle exceptions - see #12~~
 - [ ] ~~Define transaction API - see #13~~
 - [ ] ~~Make repositories unaware of the identity map (should probably use their own in-memory strategy) - see #14~~
 - [ ] ~~Remove ObjectManager API attempt and try to integrate the ORM/ODM instead - see #15~~
 - [ ] ~~Introduce `zendframework/zend-stdlib` to use hydrators for a base functionality (without persistence layer) - see #16~~