PyModule: decimal

Update information:

1. bug fixes:
   (1) Add decimals modulo ZeroDivisionError
   (2) Catch a number not in “decimal” class
2. add sum function(self.sum(var1,var2,*vars))
3. More accurate division. Uses self / value to keep 100 digits after the decimal point as a infinite decimal places. Uses decimal(1) / decimal(7) will return 0.1428571428571428571428571428571428571428571428571428571428571428571428571428571428571428571428571423. Or uses advanced function self.placediv(othernum,places,last="round"). Uses decimal(19).placediv(decimal(16),2) will return 1.19.