[fixes #108] Removes id from resource's urls.
Forgot to add migrations.
[codereview] Implemented suggestions and added a new test.

- Removed create_unique_slug method since it is not used.
- Removed unused import
- Fix problem with urls names
- Added new test
[fixes #108] Applies unique constrain to slug field.
Merge pull request #117 from openrural/feature/108-remove-id-url

[fixes #108] Removes id from resource's urls.