Add a test for failure to load config files.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Remove needless require.

The mail gem is already required at the beginning of the file, so there's no
need to demand load it in Store#each.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Add tests for Message.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Add more Message tests.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Fix inconsistent handling of string arguments to Message.

Parts of the code were changed to accept a filename as a string, but parts
of the code also expected strings to be the message itself.  Fix this
inconsistency in favor of using strings as the actual message content.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>