The start of a message store definition. Whether this will end up being used

The start of a message store definition.  Whether this will end up being used
for the archive or not is left to be seen.

Define an interface, test, and implementation of a basic message store using
globally unique identifiers of the form: archive/hash/seqno

- archive is the base url of the archive, e.g. http://archives.example.com.
  This is available in the List-Archive header.
- hash is the base32 encoded sha1 hash of the message's Message-ID and Date
  headers, which it must have.  This is available in the X-List-ID-Hash
  header.
- seqno is a sequence number specific to the archive which will uniquely
  identify the message should there be a Message-ID/Date collision.  this is
  available in the X-List-Sequence-Number header.

Added a MESSAGES_DIR variable to the config.

Added a .message_store attribute to the config.