MPD xs:string parse fix

[matt-hammond-001]

This PR is intended to fix #1573 . It adds a StringMatcher to the DASH parser.

The issue identified the specific case of Period@id attributes, however there are many other attributes in the DASH MPD that, according to the schema, are also type xs:string. StringMatcher therefore contains a mapping table listing every attribute I could find the the schema that is defined as being of type xs:string.

The StringMatcher needs to know the name of the node as well as the name of the attribute. xml2json.js was therefore modified to provide this as an argument to the test method. Although attributes currently have an ownerElement property that would provide this information (without needing to modify xml2json.js) this will eventually be deprecated in DOM4+ according to https://developer.mozilla.org/en-US/docs/Web/API/Attr

[matt-hammond-001]

d591616 adds some other attributes whose types are variants on a basic xs:string and which should be parsed as strings.

[davemevans]

LGTM.