Refactored like model integration.

Moved a few methods specific to like from Actor to SocialStream::Models::Object and added some tests.

This should also fix issue #228 by moving `after_destroy :decrement_like_count, :delete_notifications` to the earlier place of class definition and changing it to `before_destroy`, but I wasn't able to write failing model tests for that issue, so I base this statement only on my browser experiments.