Update src/jquery.multiselect.filter.js

Fixed error menu is undefined error on line 32. Changed .data('multiselect'); to .data('echMultiselect');
Added var _self = this; on line 62 of the toggleChecked override function so that the options in the original select get modified when checks get toggled.
Merge pull request #386 from davidworkman9/patch-1

Update src/jquery.multiselect.filter.js