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**Bugfixes**
- Fixed bug in `ModelFieldChecker` where node is not one of Name or Attribute (rocioar)
**Improvements**
- Added checks `DJ04` and `DJ05` for urls (rocioar)
- Reorganized code, moved individual checkers to `checkers/`, `tests to tests/` (rocioar)
- Added coverage checks (rocioar)
- Moved from Travis to CircleCI (rocioar)