Added solution using Pascal triangles

-This solution is a LOT faster since it doesn't calculate every possible
route.
-The method will calculate all required levels of the Pascal triangle
Added Problem 13
Added Problem 16 and Problem17
Added Problem 18 and 19
Added Problem20

-Added Problem 20
-Improved Math::BigInt so that it can do multiplication and addition
Added Problem 25
Changed Problem16 to use BigInt defined in Math.h