Option groups support for the dataprovider method

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
prevent the "select all" from being created twice
** fix for #282: prevent crash when trying to select non-existant value **

Signed-off-by: Tyf0x <Tyf0x@users.noreply.github.com>
Merge pull request #281 from conradfr/master

Option groups support for the dataprovider method
Merge pull request #318 from shayh/master

prevent the "select all" from being created twice
Merge pull request #319 from Tyf0x/fix_#282

** fix for #282: prevent crash when trying to select non-existant value **
#301, additional documentation.
Merge branch 'master' of https://github.com/davidstutz/bootstrap-multiselect

Conflicts:
	index-2-3.html
	index.html
	js/bootstrap-multiselect.js
#329.