Editing base64, Adding average file, Editing find_lcm (#673)

* avrage.py

calculate and print the avrage of number list.

* Update base64_cipher.py

encoding and decoding base64 without any module.

* Update and rename avrage.py to average.py

* update find_lcm algorithm

I made find_lcm more efficient form O(num1*num2) to O(min{num1,num2}).