Added RdfParser instance for XmlParser.
Haddock change, TODOs refresh, XML parser example

1. RDF, RdfSerializer & RdfParser typeclasses re-exported manually in
Data.RDF so that the haddock docs is presented at the top level on
hackage.
2. TODO.org added.
3. XML parser example added, following up from previous commit which
added an XmlParser instance of RdfParser.