* SPEC: PollFeedJob parsing atom feed
* add FeedItemAccessor
It is to provide a consistent interface to access a feed item's tag
content.
* add FeedElementInstaller
to install non-standard and non-namespaced feed elements
* FEATURE: replace SimpleRSS with Ruby RSS module
* get FinalDestination and download with Excon
* support namespaced element with FeedElementInstaller
Kyle Zhao