code: def addUnit(newUnit, newRate)
  
  			if @dictionary.nil?
  				@dictionary = Hash.new()
  			else
  				# do nothing
  			end
  
  			# certify if the key doesn't exist
  			if !@dictionary.has_key?(newUnit)
  				@dictionary[newUnit] = newRate
  
  				# verify if the key has been added
  				if @dictionary.has_key?(newUnit)
  					puts "key #{newUnit} added"
  
  					# return @dictionary
  					return true
  				else
  					# throw exception
  					return false
  				end
  
  			else
  				puts "key #{newUnit} already exists"
  				return false
  			end
  
  		end
docs: newUnit: name of the new unit to be added
   newRate: reason between new unit and base unit
   (example: kilometer it's 1000x greater than meter, so the newRate should be 1000)
   returns bool - true if succeed, false if fails