Setter for relations

[koenpunt]

@greut's work rebased.

To-do

• poly relationship option

fixes #125

[koenpunt]

@Rican7 @al-the-x is this complete as-is? Couldn't figure out what has to be changed from the discussion in #125

[Rican7]

As mentioned in #125, it would be better if we switched to checking if the object was a Model instance.

[koenpunt]

@Rican7 is this not yet the case here?

[koenpunt]

Alright, I checked out @greut's work, and I have some notes:

1. I think that we should move this functionality to the Relation class

2. We should add support for poly relationships, so the following is possible:

   $venue = Venue::first();
   $venue->events = [new Event(), new Event()];

3. maybe we can/should reuse set_relationship_from_eager_load

@Rican7 @jpfuentes2 @al-the-x @cvanschalkwijk @tuupola what do you think?