add p94, the arithemetic_puzzle

Solved problem 94:

**An arithmetic puzzle. (hard)**

Given a list of integer numbers, find a correct way of inserting arithmetic signs (operators) such that the result is a correct equation. 

Example: With the list of numbers `[2;3;5;7;11]` we can form the equations `2-3+5+7 = 11` or `2 = (3*5+7)/11` (and ten others!).