Cancan for authorization
- tasks associated with users
- users can only manage tasks that belong to them
- revamp tests old tests and add tests for devise and cancan

- https://www.pivotaltracker.com/story/show/67751402
[Finished #67751402]
Updates based on PR comments:

- #7
- https://www.pivotaltracker.com/story/show/67751402
[Fixes #67751402]
Updates based on PR comments:

- #7
- https://www.pivotaltracker.com/story/show/67751402
- [Fixes #67751402]
Merge pull request #7 from joshclick/cancan_user_auth

Cancan user auth