add base events class

Hi,

All doctrine projects implements events. A lot of them are similars, but there is no base class defining common Events.

I tried to gathen them all in this PR. I looked into ```CouchDb-orm```, ```doctrine-orm```, ```doctrine-mongodb-odm``` and ```phpcr-odm```. 

I only added events present in all packages, but CouchDB is the only one to not implement some events implemented in every other packages (```postPersist```, ```loadClassmetadata```, ```preFlush```, etc.).
Isn't this description a bit misleading here? I thought the purpose of this container is to provide common persistence events in general and not ORM specific?