Remove dependencies to RDF package

RDF is not in master yet ...
Fixed handling of property revisions

The title was not extracted correctly in this case, since the dumps
contain prefixed titles which we assumed to be titles.
* MwRevision now has both getTitle() and getPrefixedTitle()
* Tests were extended to include properties
* ItemIdValueImpl and PropertyIdValueImpl produce more useful errors
when given invalid ids.
Mention fix of issue #67.
Removed unnecessary member
Merge pull request #68 from Wikidata/issue-#67

Issue #67
Merge remote-tracking branch 'origin/master' into rdf-serializer