Set accountname in BaseFolder, and don't pass it in initialization

We passed in the accountname to all derivatives of BaseFolder, such as
IMAPFolder(...,repository,...,accountname), although it is perfectly
possible to get the accountname from the Repository(). So remove this
unneeded parameter. Each backend had to define getaccountname() (although
the function is hardly used and most accessed .accountname directly).

On the other hand BaseFolder was using getaccountname but it never defined
the function. So make the sane thing, remove all definitions from backends
and define accountname() once in Basefolder. It was made a property and not
just a (public) attribute, so it will show up in our developer
documentation as public API.

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

offlineimap/folder/LocalStatusSQLite.py

@@ -46,10 +46,9 @@ class LocalStatusSQLiteFolder(LocalStatusFolder):
     #current version of our db format
     cur_version = 1
 
-    def __init__(self, root, name, repository, accountname, config):
+    def __init__(self, root, name, repository, config):
         super(LocalStatusSQLiteFolder, self).__init__(root, name, 
                                                       repository, 
-                                                      accountname,
                                                       config)       
 
         # dblock protects against concurrent writes in same connection