Use setuptools
virtualenv: fix shebang
fix reply when multiple accounts are in replied message

In the original implementation of determine_sender, the first account that
matched a header was taken as the "To:" of the reply. It was broken when the
first account in the list was the account the mail was "Delivered-To:", not
"To:", thus shadowing the account the "To:" header.

This patch fixes the bug using the first header instead of the first account to
get the sender address.
thread: fold seen messages
thread: mark message as read on collapse
thread: add markasread, reads thread and deletes buffer.

Use case:

1. You open a thread.
2. There is on one mail, you've quickly read it and want to go back to the thread list.
3. You use :markasread, it marks the message as read and deletes the buffer.
docs: nitpick
urwidtrees: sync with upstream