Added Timeout checking to IMAP4Client

[bkirsch]

The IMAP4Client will hang if the server stop returning responses. Added timeout
to prevent this from 
happening. By default the timeout is set to 0 which means no timeout checking
will be performed. 
Set timeout to >0 for the feature to be active. This prevents unexpected
behavior from current 
IMAP4Client users.

[exarkun]

This patch causes the client to disconnect, but it doesn't do anything with any
outstanding Deferreds.  These should all have their errbacks invoked with a
timeout exception of some sort.

Also, rawDataReceived needs to call resetTimeout, like lineReceived does, since
during message body receipt, lineReceived will not be invoked.

Unit tests would be great, too.

[bkirsch]

Updated Diff that adds a timeout for rawDataReceived and errbacks any deferred commands in the 
self.queued buffer

[bkirsch]

Ok here is the final imap4.py diff.

[bkirsch]

Ok here is the test_imap.py test case diff

[bkirsch]

Added a check to make sure the lastCmd deferred has not already been called :)

[exarkun]

Committed at revision 

I made a couple minor changes to the patch before committing.  Most notably, I
suspect, I removed the string from the TimeoutError() constructor.  I think a
user-friendly messages belongs somewhere else.

[exarkun]

Committed at revision 12474

I made a couple minor changes to the patch before committing.  Most notably, I
suspect, I removed the string from the TimeoutError() constructor.  I think a
user-friendly messages belongs somewhere else.