Add namespace filters

[jgou]

We should provide namespace filters, for the following use cases:

1. strip xhtml namespace when output format is html
   • if the default namespace is xhtml, then the html tags will not be recognized (<br> is indeed not the same as <xhtml:br>) by the html serializer (and possibly all who's next in the chain, including the browser).
   • This can be a bit annoying when you write your templates in xhtml, but want to publish it as html (because IE7 doesn't support xhtml or ...).
2. add missing xmlns attributes
   • e.g. when referencing an above declared variable or ...
   • maybe have a look at the XmlSerializer in Daisy
3. drop unneccesary namespace declarations
   • e.g. the ktl-namespace isn't used anymore after leaving the template engine

[mpo]

Somewhat related to #145
Beyond 0.4 scope