URL should encode valid puzzles

[cbass10liu]

For example: (10 - 7) * 4 + 12 should encode to something like g=4,7,10,12 and 1 + 1 + 1 +1 should not encode to anything because 1,1,1,1 is not a valid puzzle (that is, no set of valid operations produces 24 given 1,1,1,1).