Core code review

- [x] Fix dogballs.
- [x] Adjust methods $ and $$ to have a similar behavior as jQuery init function.
- [x] Review comments, do not repeat target function name and make it consistent with the others.
- [x] Is `step.el` being used anywhere?
- [x] Is there a need to return anything on `var empty = function() { return false; };`?
- [x] addEventListener default last argument is set as `false`, but that is already the default value. Remove it.
- [x] Investigate meta attributes to understand more about each one of them and their function in impress