Redesigned script structure

https://github.com/cferdinandi/kraken/issues/38
Add public/private method notations
Removed `data-target` in favor of single data attribute
Updated event.preventDefault with safety checks
Convert `toggleCollapse` to a public method
Minor updates
Redesigned script structure and API

https://github.com/cferdinandi/kraken/issues/38
Launched version 5
Merge branch 'init-4'