Add optional topic to subscriptions
Allow events to be published with optional topic
Allow `.activity_feed_for` to specific an optional topic
Add "topic" examples to README
`.activity_feed_for` must not return activities with a topic if no topic is given.
Allow `:all` to be passed as topic for an activity feed which will return activities both with and without a topic.
Merge pull request #1 from krisleech/topics

Allow multiple feeds per object using topics