A simple and quick way to have feed groups in fever clients.

There are some feature requests about adding groups for feeds. Currently I'm trying to implement such feature, but I find this a bit complicated due to design, usability and other front-end issues. However, adding groups to fever API is simple and straightforward, so I thought that having groups at least in the clients would be great.

These changes do not affect UI at any way. Also I modified `opml_parser.rb` and `import_from_opml.rb` to consider groups so you can re-import the same `subscriptions.xml` just to attach existing feeds to groups.

Tested on Reeder for iPhone.