creating default output for transitive reduction for owl:Thing to be
used when there are no (direct) subsumers found
Merge branch 'hotfix/issue_24'

Conflicts:
	elk-reasoner/src/main/java/org/semanticweb/elk/reasoner/reduction/TransitiveReductionFactory.java
small fix for conditions of using defaultTopOutput as direct subsumers:
owl:Thing might be in the equivalence class
class taxonomy and instance taxonomy factories do not need references to
the top node anymore because the respective equivalence class for top
will be created by transitive reduction
Merge branch 'hotfix/issue_24'

Conflicts:
	elk-reasoner/src/main/java/org/semanticweb/elk/reasoner/reduction/TransitiveReductionFactory.java