http://*.com/questions/7121213/singleton-instantiation
http://docs.oracle.com/javase/specs/jls/se8/html/jls-8.html#jls-8.4.3.2
http://docs.oracle.com/javase/specs/jls/se8/html/jls-12.html#jls-12.4
http://docs.oracle.com/javase/specs/jls/se8/html/jls-14.html#jls-Block
http://docs.oracle.com/javase/specs/jls/se8/html/jls-13.html#jls-13.4.25
http://www.journaldev.com/1377/java-singleton-design-pattern-best-practices-examples