Imbox() usable as with statement

[sblondon]

This allows usage like:

with imbox.Imbox("imap.server.domain.tld", "user", "password") as mbox:
... mbox.folders()

and the logoout() is made automatically.

The patch update the README to use it.

This probably my last patch so don't wait others ones if you want to release a new version.

Thanks for all the quick merges!

[martinrusev]

@sblondon Thanks for the contributions. This one is really nice. Merging and pushing to pypi