This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
* 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
