Support associations to reduce calls to server

[sthaha]

The following code

release = Release.find('fedora-22')
release.variants.map(&:release) 

Fetches release X times instead of 0. Perhaps the caching may stop this from happening but it may be worth fixing by adding support of association.

[ycheng-aa]

@sthaha Does it mean we should port Spyke association implementation to PDC gem?

[sthaha]

@ycheng-aa yes, most of the gem is based on spyke which already has associations. We could base our implementation on spyke.