NNTP Cleanup

Implements a number of changes to the NNTP API outlined in #894

- [x] Clarify the meaning of "subscribed"
- [x] A faster way to build an article list
    - [x] Fetch Indexes
    - [x] XOVER
- [x] Message-ID support
- <strike>Newsgroups formed after a certain date.</strike>
The class should probably be renamed to to NNTPFetchAllArticlesOperation.