Merge pull request #521 from tonioo/1.1.x

Update imap_and_smtp.rst
Merge pull request #524 from tonioo/1.1.x

1.1.x
Merge branch 'master' of github.com:tonioo/modoboa
Merge branch '1.1.x'
Merge branch '1.1.x'
Merge branch '1.1.x'
Merge branch '1.1.x'
Merge branch '1.1.x'
Update plugin_api.rst

Import for ModoExtension was needs to be from modoboa.core.extensions.
Merge pull request #529 from jag3773/patch-1

Update plugin_api.rst