12 Integer to Roman

Given an integer, convert it to a roman numeral.

Input is guaranteed to be within the range from 1 to 3999.

Thoughts:
这道题主要就在于如何处理每一位digit，并且按照区间不同处理：
1<=digit <=3
digit =4
digit = 5
5<digit<=8
digit =9
credits: http://bit.ly/1OmnkR0

Python Codes: