readded support for computing referrrer changesets (fixes #169 and reverts 8e57615d304)
readded support for computing referrrer changesets (fixes #169 and reverts 8e57615d304)

it should also be noted that we now ensure that we only compute a changeset once, so the issue we tried to fix with 8e57615d304 isnt so severe anymore

also prevent initialization reference/referrer collections just to compute change sets

doctrine/phpcr-odm,Ocramius/phpcr-odm,AxaliaN/phpcr-odm2,uwej711/phpcr-odm,doctrine/phpcr-odm,sarcher/phpcr-odm,valiton-forks/phpcr-odm,AxaliaN/phpcr-odm2,WouterJ/phpcr-odm,uwej711/phpcr-odm,Ocramius/phpcr-odm,WouterJ/phpcr-odm,sarcher/phpcr-odm,valiton-forks/phpcr-odm