Added tests for postman.daemon.Postman
Added a method to postman.daemon.Postman to "clear" all the storage associated
to a given postman instance (removing all the files from disk)
Fixed some bugs, mostly caused by typos through the sources, detected while
writing more tests
Removed the storage and archive attributes from postman.models.MailingList,
now they are methods "marked" as properties using the @property decorator.
This keeps the storage objects from being serialized into json objects by
the storage backend (de-serializing them caused some trouble and it is not
necessary at all)
Added create() and delete() methods to postman.storage.JsonStorage. the first
one ensures the path to the storage file exists (creating all subdirs if
necessary) and the second one deletes the storage file from disk
