Association DSL should raise arg error when target relation is missing a PK

[solnic]

Refs #137

[GustavoCaso]

I having a look at this one 😄

[solnic]

This won't make it into 2.0.0. I changed the milestone to 2.1.0. My plan was to introduce a nice validation solution for association setup, as it's easy to mess up association definitions. This will be a bigger effort of course, but I think it's worth to do it.