Added `loop` as an option
Added new methods, improved performance and updated readme.md

- New isFirst method
- New isLast method
- Improved performance removing repetitive calls to getCurrent method
- Updated readme file with information about isFirst and isLast
- Now onInit and onSwitch both have params to access the api, check the readme for details

This commit proudly closes #3
Minor style changes to the demo.html and demostyles.css

- Moved prev and next buttons to top
- `Lorem ipsum` now is in italic