Improve namespace handling:
  - XML::Element.(nsURI || nsPrefix) check parent elements for xmlns declarations.
  - XML::Element.nsPrefix recognises only xmlns or xmlns:* attributes as namespace declarations.
  - XML::Element.elements :URI test won't match undeclared URIs.
Add test for :URI on (n > 1)th level descendants
Merge pull request #9 from fjwhittle/improve-ns-handling

Improve namespace handling