Improve tests

- [x] added groups in Guardfile for rails and specs (launch with bundle exec guard -g rails or -g specs)
- [x] added user and proof models specs
- [x] improved coverage results with :rails parameter
- [x] added proofs controller specs
- [x] added requirements_helper specs
- [x] general cleanup and renaming of requirements feature specs
- [x] added proof attach image feature specs