Implements coverage tests and code analysis

[aleDsz]

Changelog

• Add ExCoveralls library to implement code coverage
• Add more tests to increase code coverage up to 100%
• Implements TDD library (optional)
• Remove unused functions to reduce warnings
• Add fixtures to use with specific tests
• Add credo file to improve code analysis

Closes #5.

[samuelpordeus]

@aleDsz I'm waiting for it to complete 1 year so we can have a cake party for the PR! 🍰