Bug 800402 - [email] UI: Show the number of unread messages in the message-list header after the folder name

for coverage purposes, we also really want to make sure that:
- we have tested fetching subsequent batches.  So I think this suggests we want to modify things so the batching constant is something we can clobbe here in our test.  Ideally it would fire a few times.

- we want to make sure we're calling flushExcessBlocks.  So let's have our mock or whatever implement the method (also required so we don't explode) and somehow also have a counter.  And we expect that counter to (18/BATCH_SIZE - 1) or whatever it works out to be.