Remove unused import.
Disable not partial updates on attachment resources.

PUT request does not supports multipart request, but attached_file
is required. Because of this condition, put requests for attachments
resources are not possible, for it, them are disabled explicitly.
Check if project matches the content_object.project.
Remove deprecated attributes on attachments resources.
Remove unsed import on attachments permissions module.
Set attached_file field on attachments serializer required.
Remove usage of django-testclientextensions (backport of some commits from django1.7 branch)
Fix affected tests to previous attachment fixes.
Replace direct JSONRender usage with taiga own json module on all tests.

So unify the json interface in all tests code.
Merge pull request #65 from taigaio/bug/attachments

Attachments related bugfixes