Fix pickling error with AsyncResult
prev commit: do not pickle app
celery crashes on git pull

If tasks.py file is modified in git repository and I run git pull on server, then celery crashes.

"Git pull" first removes the file and then restores it, but pyinotify catches removal event.

I updated _maybe_modified function to first verify if file exists and if not then mark file as "unmodified", because afterwards will be another event when file is recreated.
Update monitoring.rst

fixed the doc on how to change flower port.
Added information on how to connect to a distant broker.

Informations found on https://github.com/mher/flower#usage,
I thought it was useful to have it in this doc too,
as it is quite necessary for the flower use.