maildir: Create top level dir recursively

This patch fixes the situation when "localfolders" specifies path that
is more that one level deep and top directory does not exists.  Example
would be "localfolders = ~/Mail/a".  This especially relevant on the
first run.

In that case we would end up with unhandled exception causing
unexpected termination of the program.

Thread 'Account sync test' terminated with exception:
Traceback (most recent call last):
  File "/offlineimap/offlineimap/threadutil.py", line 172, in run
    Thread.run(self)
  File "/usr/lib/python2.7/threading.py", line 754, in run
    self.__target(*self.__args, **self.__kwargs)
  File "/offlineimap/offlineimap/accounts.py", line 258, in syncrunner
    self.localrepos  = Repository(self, 'local')
  File "/offlineimap/offlineimap/repository/__init__.py", line 82, in __new__
    return repo(name, account)
  File "/offlineimap/offlineimap/repository/Maildir.py", line 40, in __init__
    os.mkdir(self.root, 0o700)
OSError: [Errno 2] No such file or directory: '/Mail/a'

By replacing call to "mkdir" with "makedirs" we can simply create
directories recursively.

Signed-off-by: Łukasz Żarnowiecki <dolohow@outlook.com>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/repository/Maildir.py

@@ -37,7 +37,7 @@ class MaildirRepository(BaseRepository):
 
         # Create the top-level folder if it doesn't exist
         if not os.path.isdir(self.root):
-            os.mkdir(self.root, 0o700)
+            os.makedirs(self.root, 0o700)
 
         # Create the keyword->char mapping
         self.keyword2char = dict()