Store info of user that created an invitation
Merge pull request #60 from taigaio/invitations

Store info of user that created an invitation
Minor fixes.
Tasks: class-based to function-based services
USs: class-based to function-based services
WIP: Simple proposal of permission management
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Import some rest framework parts with own changes.
Add views.py
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WIp
Adding first base_api_permissions test