%task12-1 - Added solution/rpn_calculator.rb
%task12-2 - Added method push(value)
%task12-3 - Added methods plus,value but got nil
%task12-4 - Added initialization method
%task12-5 - Added method plus,value
%task12-6 - Added method minus
%task12-7 - Added methods divide,times
%task12-8 - Added method tokens
%task12-8 - Wrote method tokens(used split.map,.to_sym)
%task12-9 - added method evaluate
%task12-10 - Wrote method evaluate, but need return result
%task12-11 - Added method value in the end of method evaluate for result