fixed randomNumber's frontwards bias (#26)

Before in randomNumber return crypto.randomBytes(1)[0] % max;
was used to generate the random number. Using modulo at the
end does lead to a bias in the probability distribution of the
generated characters if the character count does not divide 256.
E.g. if using randomNumber(255) the number 0 is twice as likely
as all other numbers and thus the character 'a' will appear
much more frequently.

The fix is to discard the random value if it is above the highest
mulitple of the character count that is not greater than 256.

src/generate.js

@@ -5,7 +5,11 @@ var self = module.exports;
 // Generates a random number
 var randomNumber = function(max) {
 	// gives a number between 0 (inclusive) and max (exclusive)
-	return crypto.randomBytes(1)[0] % max;
+	var rand = crypto.randomBytes(1)[0];
+	while (rand >= 256 - (256 % max)) {
+		rand = crypto.randomBytes(1)[0];
+	}
+	return rand % max;
 };
 
 // Possible combinations