More useful aliases

* `ActivityFeed.for` as an alias for `ActivityFeed.feed`
* `ActivityFeed.between` as an alias for `ActivityFeed.feed_between_timestamps`
* `ActivityFeed.trim` as an alias for `ActivityFeed.trim_feed`
* `ActivityFeed.expire_in` and `ActivityFeed.expire_feed_in` as aliases for `ActivityFeed.expire_feed`
* `ActivityFeed.expire_at` as an alias for `ActivityFeed.expire_feed_at`
Merge pull request #17 from agoragames/more-aliases

More useful aliases