Fixes issue #474 - Checkbox and radio button groups using custom namespace

In `ParsleyFieldMultiple.initMultiple()` there is the following line...

          this.siblings = this.group ? '[parsley-group="' + this.group + '"]' : 'input[name="' + this.$element.attr( 'name' ) + '"]';

which I think should be using the `options.namespace` value rather than the hardcoded `parsley-group`...

          this.siblings = this.group ? '[' + options.namespace + 'group="' + this.group + '"]' : 'input[name="' + this.$element.attr( 'name' ) + '"]';