[WIP] API cleanup

[Ocramius]

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

[coveralls]

Coverage decreased (-0%) when pulling 50dc168536242018c400d2a792bbae5a179a0093 on hotfix/api-cleanup into d5c316f8708add0396e1ba4d36aa77e549b8da06 on master.