Refactoring toggler

We've decided to get rid of the `toggle`, `disable` and `enable` events. There are methods of toggle instead. Every method in addition fires the events: 'nb-toggler_checked', 'nb-toggler_disabled' and 'nb-toggler_enabled' accordingly.

Also there is a chance to toggle manually using the `setValue` method and a public getter for the state's value, so you can check it.