allow misc. namespaces in XHTML

**This PR is intended to open a _discussion_ on options for allowing namespaces in generated XHTML.**

The `xml:lang` attribute is [required on the html element](http://www.w3.org/TR/xhtml1/#C_7) for XHTML documents and [correct language specification](http://www.w3.org/TR/UNDERSTANDING-WCAG20/meaning-doc-lang-id.html) is also a Level A accessibility requirements for WCAG 2.0

This experimental change preserves all namespaces but those with the epp: and epc: prefixes in generated XHTML, and mangles the xhtml: prefix.

**Note well:** a proper implementation would have to _at least_ inspect the namespace definitions to discover the prefixes for namespaces whose URIs are `http://eprints.org/ep3/phrase` and `http://eprints.org/ep3/control`, rather than assuming epp: and epc: