imaplib2 debugging

[Piers Lauder]

I have committed a new version of imaplib2 with extra logging, and a
new paremeter to the class 'debug_buf_lvl' (default: 3), that controls
log buffering.

So, to invoke IMAP4 with output to stderr at logging level 3, one would
invoke as:
  IMAP4(..., debug=3, debug_buf_lvl=2)

More lock traces now come out at debug level 3, which involved much less
output than level 5 (which traces actual I/O).

Hope that helps...