Test that the div in XHTML content is in the correct namespace.
Check the namespace of XHTML divs.
Warn doubly for a non-XHTML-namespaced div.

Warn that an xhtml:div is missing and that the 'div' is in an
unexpected namespace when content's top-level 'div' has an
incorrect namespace.
Merge pull request #4 from josephw/xhtml-div-checking

Check the namespace for divs in XHTML content.