only use disabledText when multiselect is disabled

Fixed collapisble optgroups and added tests for correct mirroring and initialization of clickable optgroups.

fix optgroup uncheck accidentally unchecking its above neighbors

Implement data attributes

Fixed documentation

Added better knockout documentation.

website overflow issues fixed

Fixes #714

Added tests to clarify that selectedOptions is what
 to use when you want to update the checked state of
 multiple options.

Fixed #575. The select all is not triggering the onChange anymore. Only the onSelectAll is triggered.