[#888] implement 'multiple' property in questions

* at the moment, this property is only shown in barcode questions.
[#888] Use allowMultiple flag instead
[#888] incorporate pull request comments

* delete xmlSchemaType - NMTOKEN
* change way of checking null on getAllowMultipleFlag
Merge pull request #889 from akvo/feature/888-barcode-reader

[#888] implement 'multiple' property in questions