Corrects `update` when no items exist in loop mode
Disables navigation controls when no navigation is possible

Instead of using the `display` attribute the class `disabled` is used
whenever a control container is disabled. Thus the user has more control
how this should be displayed.

Closes #323
Corrects typo in items `active` and `center` classes
Corrects variable names for settings in navigation

Since 'options' stands for all non-adaptive settings, some variable
names need to be adapted in the navigation plugin.
Merge branch 'develop' of https://github.com/OwlFonk/OwlCarousel2 into autoplay-buttons