implementation of smtp mail server queue for "--no-server" option

Configuration:

1. Configure following options in settings.py

```
LETTUCE_SMTP_QUEUE_HOST
LETTUCE_SMTP_QUEUE_PORT
```

this is where lettuce smtp server will be listen to for incoming email messages.

2. django application smtp settings should be configured accordingly for sending emails to lettuce smtp

```
EMAIL_HOST
EMAIL_PORT
EMAIL_HOST_USER = None
EMAIL_HOST_PASSWORD = None
```

3. Run django app

4. Run lettuce

```
./manage.py harvest --no-server --smtp-queue
```

This plugin use lettuce.django.email.queue , but as it parse incoming smtp emailIs to django EmailMessage, there can be some differences in emails (emails usually won't be fully equal in all headers, attachments, etc, but it works ok for simple cases like checking body and common headers). I was trying to write some tests for plugin but with no luck.