Create "baseimages" directory if does not exist at startup

[Marco Giusti]

Here the traceback of the command "i files" from the console. Maybe other parts are broken too.

An exception is occurred while processing command i files
Traceback (most recent call last):
  File "/home/marco/.virtualenvs/virtualbricks/src/vb-debug/virtualbricks/brickfactory.py", line 622, in _poll
    console.parse(self.factory, command)
  File "/home/marco/.virtualenvs/virtualbricks/src/vb-debug/virtualbricks/console.py", line 310, in parse
    return protocol.lineReceived(command)
  File "/home/marco/.virtualenvs/virtualbricks/src/vb-debug/virtualbricks/console.py", line 330, in lineReceived
    handler(parts[1:])
  File "/home/marco/.virtualenvs/virtualbricks/src/vb-debug/virtualbricks/console.py", line 436, in do_images
    self.sub_protocols["images"].lineReceived(" ".join(args))
  File "/home/marco/.virtualenvs/virtualbricks/src/vb-debug/virtualbricks/console.py", line 330, in lineReceived
    handler(parts[1:])
  File "/home/marco/.virtualenvs/virtualbricks/src/vb-debug/virtualbricks/console.py", line 552, in do_files
    for image_file in os.listdir(dirname):
OSError: [Errno 2] File o directory non esistente: '/home/marco/.virtualbricks/img'