Aligned the Event classes to match those of the ORM

fixes https://github.com/doctrine/phpcr-odm-documentation/issues/60
:+1: for @dbu Why having one specific argument for each event? There are only two different use cases: 
- one event with `getObject` and `getObjectManager` for all except the flush events (ODM makes some special caused by documents)
- one event for the flush stuff

The hardest thing in writing listeners is the "EventArgs-Bingo" - which is the right one and for which doctrine?