Add class AttributeValue that fixes onelogin#117
AttributeValue is a String, but `AttributeValue#values` will return
all values for that attribute.
Fixes onelogin#112 by extracting all Attribute values into single array
Require AttributeValue class in correct place
Return attributes always in reverse-XML order
Merge pull request #122 from borgand/multiple-attribute-values

Fixes #112 and #117 in a backwards compatible manner