Code cleanup around ES6

[aarranz]

Proposed changes

This PR makes some code clean ups, like configuring eslint to use the no-var and prefer-const options and improve the usage of ES6 classes on the code.

Types of changes

• [x] Refactoring (non-breaking change which improves code quality without changing the current behaviour)
• [ ] Bugfix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• [x] I have read the CONTRIBUTING doc
• [ ] I have signed the CLA
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added necessary documentation (if appropriate)
• [x] Any dependent changes have been merged and published in downstream modules

Further comments

This PR does not solve al the problems related to ES6 adherence, the rest of the needed clean ups will be addressed on further PR.

[coveralls]

Coverage increased (+0.1%) to 88.595% when pulling 7be2a4a8e111ee496ec667c2d8ff8f08d4aa45a5 on aarranz:task/es6-classes-cleanup into 5943d56c6e88e419868c9be7f4627244140a8c39 on Wirecloud:develop.