Batch header requests to speed up the process

Running time on about 6000 messages went from 11 minutes to 18 seconds
by batching per 100 messages.

When requesting all headers at the same time the duration was 11 seconds,
so reducing the size of the batches further seems to not be worth the
reduction in user feedback.
Merge pull request #8 from bjorn/master

Batch header requests to speed up the process