#184: fixed subscription to Notifications collection

This fixes #184 . Somewhy in 9c4f030b909111df0b9acfc7e22b94049a3ff0a8 `Meteor.userId()` was changed by `Meteor.user()` check, which may be null until profile is loaded. That's why notifications broke.

One more problem that I've fixed: if user wasn't logged in and the he signed in, we should notice that and subscribe to notifications. That's why I wrapped this call into `Deps.autorun`