Remove dangling event listeners on frame removal.
Remove listener cleanup from ping failure handler.
Add codeclimate badge.
Make answers more user-friendly.
Update comments for user-friendly answers.
Remove unnecessary "tabs" permission.
Bump version to 0.6.4
Remove changelog from readme.
Add link to the releases page.
Bump version to 0.6.4
Merge branch 'develop'

- Make answers more user-friendly, e.g. "second (1/60 minute)" can also be answered as "second".
- Remove unnecessary "tabs" permission.
- Fix dangling event listener on frame removal.