[WIP] NNTP Cleanup

Implements a number of changes to the NNTP API outlined in #894

1. [x] Clarify the meaning of "subscribed"
2. [ ] A faster way to build an article list
    - [x] Fetch Indexes
    - [x] XOVER
    - [ ] XOVER Single
3. [x] Message-ID support
4. [ ] Newsgroups formed after a certain date.
I'd suggest:

```
void NNTPFetchOverviewOperation::main()
{
    ErrorCode error;
    mArticles = Array::array();
    for(unsigned int i = 0 ; i < mIndexes->rangesCount() ; i ++) {
        Range range = mIndexes->allRanges()[i];
        Array * articles = session()->session()->fetchOverArticlesInRange(range, mGroupName, &error);
        if (error != ErrorNone) {
            setError(error);
            return;
        }
        mArticles->addObjectsFromArray(articles);
    }
}
```