Multiple Decision Profile - Replace xml:id with Id

[cdanger]

The following changes introduced in ACAL 1.0 / XACML 4.0 core spec result in changes to the Multiple Decision Profile that require an update of the Profile's specification:

1. As suggested in Remove Attribute's IncludeInResult from Result and PolicyIssuer #18, the xml:id XML attribute should become an unqualified attribute like Id for the sake of the JSON and YAML representations.
2. Fixing issue Remove Attribute's IncludeInResult from Result and PolicyIssuer #18 renamed the Request's Attributes element to RequestCategory, therefore the AttributesReference is renamed to RequestCategoryReference for consistency.