[twgit] Init feature 'feature-unit-tests-for-exec-fct'.
Unit tests: 
  - covers Helpers::exec()
Doc:
  - Add URL of coveralls.io badge on README.md.
  - Code is now PSR-2 compliant.
[twgit] Init release 'release-1.2.0'.
Merge branch 'feature-unit-tests-for-exec-fct' into release-1.2.0
Update CHANGELOG.md.
Merge branch 'release-1.2.0' into stable