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
Documentation fixes.
IMG_ROOTDIR paramter removed.

fix #530
Fix attempt...

see #534
Bad import statement fixed.

fix #538
Additional checks on account import.

fix #539
Ignore empty recipients.

fix #540
Do nothing if the user is anonymous.

fixes #548
CKeditor compatibility.

see #547
Don't try to use mbox if it is not defined.
Forward a message AND its attachments.

Code cleanup.

see #545
Wrong import statements fixed.

see #545
Only decode when necessary.

fix #545
Disallow # character in filter names.

fix #550
Adapt identities state based on domain's state.

fix #542
New Dutch translation.

fix #528
New Dutch translation.

fix #528
Merge branch 'master' of github.com:tonioo/modoboa
Add current version to the User-Agent header.