Make str() of Repository() be its name

So we can simply hand a repository instance to error messages rather than
having to call Repository().getname() all the time.

This is not used yet.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/repository/Base.py

@@ -72,6 +72,9 @@ class BaseRepository(CustomConfig.ConfigHelperMixin):
     def getname(self):
         return self.name
 
+    def __str__(self):
+        return self.name
+
     def getuiddir(self):
         return self.uiddir