Addition of missing XMP standard value types

Not all value types, defined in XMP Specification, are included in org.apache.padaf.xmpbox.parser.NSMapping. Thus its method isBasic() returns false on such types, like Choice or Real.
If PDF/A file includes custom metadata and its extension schema description with such types, PDF/A validation using Preflight fails with error like this:
"7.6 : Error on MetaData, ValueType 'closed Choice of Text' is unknown. no declaration found in this schema"

These value types are not included and regarded in NSMapping class:
• Basic value types: (open and closed) Choice, Dimensions, Locale, MIMEType, ProperName, Real, Thumbnail, XPath
• Media Management value types: AgentName, RenditionClass, ResourceEvent, ResourceRef, Version
• Basic Job/Workflow value type: Job
• EXIF schema value types: Flash, CFAPattern, DeviceSettings, GPSCoordinate, OECF/SFR, Rational

Note, that XPath type in NSMapping is included only in conjunction with bag, but spelled incorrectly as "bag Xpath".