Add a factorial function

[swinton]

It would be cool if the calculator provided a way to calculate factorials, e.g.

n	n!
0	1
1	1
2	2
3	6
4	24

It looks like we just need to expose a new factorial function here:

calculator/api/controllers/arithmeticController.js

Lines 13 to 18 in a1c403b

	var operations = {
	'add': function(a,b) { return +a + +b },
	'subtract': function(a,b) { return a - b },
	'multiply': function(a,b) { return a * b },
	'divide': function(a,b) { return a / b },
	};

---

[swinton]

OK, let's estimate how big of a task this will be, using T-shirt sizing.

/polls Small Medium Large XL

[swinton]

/polls Small Medium Large XL

[swinton]

/polls Option1 'Option 2' "Option 3"