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.).
@stof thinking about this again I think it actually will break existing Events class because in the nex step those are intended to inherit from this new base events class. Or do you want to preserve the old ones and add additional constants that follow PSR?