slimming
diet continuing
Exceptions cleanup, As<Entity> interfaces
cleaned up some warnings
IsAnonymous interface and default value
cleanup on getName
some repeated method implementation moved to abstract classes as default
Dropped string cache in favor of String.intern()

In Java 8 (and recent enough 7) String.intern() does not hog memory like
in Java 6 and it's faster and better than local pools.
http://java-performance.info/string-intern-in-java-6-7-8/
getAxiom cleanup
isLogicalAxiom/isAnnotationAxiom methods cleanup
various minor simplifications
top/bottom object/data properties
isData/Object expression cleanup
top/bottom entity methods cleanup
replace Google Optional with Java 8 Optional (#250)
clean up visitor on ontology change and change data
missed @Nonnull annotations on accept methods
visitor cleanup
SWRL visitor cleanup
axiom visitor cleanup