Fix a series of issues introduced by Protégé and some weird imports...

Issues spotted by Niklas that have to be fixed:

1. The SKOS namespace is still the non-reverted ("2008") one.
According to <http://www.w3.org/TR/skos-reference/#vocab>, it should
be <http://www.w3.org/2004/02/skos/core#>. (The reversion is described
e.g. at <http://www.w3.org/TR/2009/CR-skos-reference-20090317/#changes>.)
Shouldn't this be changed in bibo (or is bibo "locked" to the 2008
draft..)?

2. Every use of rdfs:isDefinedBy (and the two rdfs:seeAlso) have
xsd:anyURI-datatyped literals as objects instead of actual URI:s.
Isn't that an error?

3. The (bibo) "abstract" property is also stated to be
rdfs:isDefinedBy "http://purl.org/dc/terms/". I suspect it should be
<http://purl.org/ontology/bibo/>?

4. There are a lot of statements about terms from other vocabularies.
I suspect many are "imported" for reasoners and/or Protegé to
function? But at least rdf:value being an owl:ObjectProperty seems
wrong, doesn't it?