Add 2 new slides about overloading of operators

Listed all the operators, which can be redefined. Used information from the following sources:
http://ruby.about.com/od/oo/ss/Overloading-Operators.htm
http://phrogz.net/programmingruby/language.html#table_18.4

Also corrected the outdated list of instance methods of Enumerable (lazy was missing).