Create ZigZagConversion.py

#The algorithom is quite simple, but it took much longer time to be accepted than I expected.
#The first few version always trigger TLE. The problem is that I used a extra dictionary at first. Moreover, it is very slow to use string + string to create new string. It is better to use ''.join from list to create a new string. It is easy, cos in python the string content cannot be manipulated.
xrange() is faster than range()