Option groups support for the dataprovider method

** Keyboard navigation fix: disabled elements are no longer selectable *...

** Removed the mirroring on the "Select All" option. **

Signed-off-by: Tyf0x <Tyf0x@users.noreply.github.com>

Signed-off-by: Tyf0x <Tyf0x@users.noreply.github.com>

Moving down the onChange callback

** Performance improvement ** - based on thorst #268 contribution.

adding namespace to fix keydown event.

Update bootstrap-multiselect.js

Fix multiple divider on rebuild().

Added Array forEach function for browsers that don't support EcmaScript5 (IE 8/9)

-> The deselectall function was deselecting all options regardless of their visibility. It now takes it into account.

-> Fixed a bug where disabled options where incorrectly selected using the 'select-all'
-> Changed the jQuery .not() selector in favor of the css native :not() to slightly improve performance

Divider was not removed on rebuild so each rebuild would add a new divider.

Moving down the onChange callback after this.updateSelectAll() allows to access the correct selected values during the callback.

Use the same format as Select2 : [{children: array (of options), [title: string]}]

Additionally :
- if no title is given, use the generic "Group n", with n as group index.
- for options if no label is given, use value as label

-> Now only filters the options to select if not all them have to be selected. This means to select all should be faster when there is no filter used.

-> improved performances and fixed the bug where the user couldn't deselect all in filtered mode
-> added the class multiselect-item to all plugin-specific items
-> fixed an incorrect CSS selector
Signed-off-by: Tyf0x <Tyf0x@users.noreply.github.com>

Remove/clear multiselect instance from select element upon calling 'destroy'.

Select-All performance improvement.

Remove multiple $(event.target) object creation.

-> Drastic performance improvement to the "Select All" button

Conflicts:
	js/bootstrap-multiselect.js

#270, #263, #260, #262, #261, #235. New option: includeSelectAllIfMoreThan. Fixed issues with select all and dividers. Updated documentation concerning show/hide event.