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