Feed XML now is downloaded using RestClient gem, instead of Feedzirra::Feed.fetch_raw.

RestClient has much better support for working with HTTP headers. This will allow us to retrieve only updated feeds (through the 'if-none-match' and 'if-modified-since' HTTP headers) even across reboots. The Feedzirra::Feed.update method does not work after reboots, which is a huge limitation.

From now on we will use Feedzirra only for its feed-parsing capabilities.