[v3] Use Jest for tests, added code coverage reports

[carlosrberto]

Changes in this Pull Request

• Replace Karma with Jest
• Added code coverage report with jest --coverage (yarn cov)
• Run a static-server to preview code coverage reports (yarn cov:serve)
• Publish coverage report to coveralls (I've update .travis.yml file)

This pull request closes #65.

[jhonmike]

please remove karma.conf.js

[carlosrberto]

I'll fix these points.