[4.2] Addition of findOrNew, firstOrNew, firstOrCreate, updateOrCreate to relevant relations.

I make extensive use of the ```findOrNew```, ```firstOrNew```, ```firstOrCreate```, ```updateOrCreate``` methods in the Eloquent model but am missing them in the ```BelongsToMany```, ```HasOneOrMany``` and ```MorphOneOrMany``` relations. I've added them myself and will write the tests if there is interest in taking this on. Thank you for considering!