Provide IInboxes as a tool
Add Inboxes.send_message() as a convenience method

It creates the messages and if needed inboxes and
conversations.
Add IMessagingLocator.get_inboxes(), tests

Add .get_inboxes() to return the actual inboxes storage.

Test if the tool is installed and MessagingLocator.get_inboxes() returns
the tool.
.gitignore var/
Fix Inboxes to remove the acquisition context

This won't work outside tests when we get and aq wrapped Inboxes
object
Extend doc, add doctest