Added attachment model + some tests

Ref #24
Add attachments on email sending
Model cleanup
Rename attachment fields

original_filename -> name
attached_file -> file
Raise error if send_many() is used with attachments
Optimization - prefect_related('attachments') in get_queued()
Fix Python 3 test
Test for utils.add_attachments
Test mail.send() with attachments
Test dispatch() with attachments
Fix test for Python3
Attachment support in EmailBackend
Refactoring - use mail.create() method in backend
Fix Python 2.6 and Python 3+
Remove attachment support from utils.send_mail()
Support passing filename string to add_attachments()
Close file afterwards
Save attachments as uuid4 + ext
Make email:attachment and m2m relation
Support adding the same "attachment" objects to multiple mails