Implement the "createfolders" setting for repositories

By default OfflineImap propagates new folders in both
directions. Sometimes this is not what you want. E.g. you might want
new folders on your IMAP server to propagate to your local MailDir,
but not the other way around. The 'readonly' setting on a repository
will not help here, as it prevents any change from occuring on that
repository. This is what the `createfolders` setting is for. By
default it is `True`, meaning that new folders can be created on this
repository. To prevent folders from ever being created on a
repository, set this to `False`. If you set this to False on the
REMOTE repository, you will not have to create the `Reverse
nametrans`_ rules on the LOCAL repository.

Also implement a test for this

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>

offlineimap.conf

@@ -504,6 +504,14 @@ remoteuser = username
 # one.  For example:
 # folderincludes = ['debian.user', 'debian.personal']
 
+
+# If you do not want to have any folders created on this repository,
+# set the createfolders variable to False, the default is True. Using
+# this feature you can e.g. disable the propagation of new folders to
+# the new repository.
+#createfolders = True
+
+
 # You can specify 'foldersort' to determine how folders are sorted.
 # This affects order of synchronization and mbnames. The expression
 # should return -1, 0, or 1, as the default Python cmp() does.  The two