Rename subscriptions association to notification_subscriptions

[apuntovanini]

Issue #161:

Summary

Changed default association name between notification and subscriptions into has_many :notification_subscriptions from has_many :subscriptions in order to avoid name clashes.

Other Information

I tried to make the association name configurable, but could't kind a proper way. Notification_subscriptions is opinionated just like subscription was, but I think it's a better default (every SaaS would have a subscription.rb model for instance that would clash with this)