XML writing

- implement writeXML method in XML class
  + write an xml file based on the data in the data structure
- do not write data to the generated XML if a default value was set
  programmatically
- add a "namedCollection" element to the XML
  + this is in preparation for the removal of opensusePattern and rhelGroup
  + the KIWIXMLPackageCollection class already writes the <namedCollection>
    element as there is no way to differentiate one collection from the
    other <opensusePattern> vs. <rhelGroup>. Therefore, the schema has to
    support the new element name prior to the old collection names
    being removed.
- alpha sort the element declarations in the schema
  + benign change, but it aides XML code development
- kiwi now depends on File::Slurp
- implement a writeTest script that can be used to verify that round
  tripping of XML data works
- create a sample config.xml file for round trip testing