DCOM-250: [GH-335] add base events class

[doctrinebot]

Jira issue originally created by user @doctrinebot:

This issue is created automatically through a Github pull request on behalf of jdeniau:

Url: https://github.com/doctrine/common/pull/335

Message:

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.).

[doctrinebot]

Comment created by @doctrinebot:

A related Github Pull-Request [GH-335] was assigned: https://github.com/doctrine/common/pull/335