[Patch] remember() for relations in Eloquent\Builder

I found out that `remember()` works only with model query builder. Related models are not cached.

This patch provides the functionality for caching related models queries.

@taylorotwell could You take a minute and review it?
I'm not sure what to do with parent model cache key. 

For now related models won't get cache key from parent (because it will conflict with cache data).