Added dependency on the Feedzirra gem.

It seems latest stable version (0.1.3) is not compatible with latest stable Rails (3.2.13); they have conflicting dependency versions. I'm fetching Feedzirra v.0.2.0.rc2 from github, it was tagged almost a year ago. This version is compatible with latest stable Rails.

I'm forced to look for a feed parsing gem because the Atom parser included in Ruby stdlib throws an errow when parsing an Atom feed without an author node (like for instance the extremely popular XKCD atom feed at http://www.xkcd.com/atom.xml). From the specification it seems that element is mandatory, but I cannot afford to be incompatible with a popular feed just because it misses that tag.
Migrated all code from using the stdlib RSS module to using the Feedzirra gem.
During unit testing of the Feed model, only the class that fetches the XML of a feed is mocked (so that no HTTP call is actually made). The parsing of the XML is done with the actual Feedzirra::Feed class, not with a mock.
The Feed unit test also tests with a mock XML for an Atom feed (before this commit, it was only testing with a mock XML for an RSS 2.0 feed).

Maybe in the future we should add mock data for other supported formats to the test.
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.