Added combined comparison operator.

Hi,

I've added combined comparison operator (<=>), which present in Ruby. It would be useful in `.sort()` method of array.

The following code...
```coffeescript
[ 1, 3, 2 ].sort ( left, right ) -> left <=> right
```

is equivalent to...
```coffeescript
[ 1, 3, 2 ].sort ( left, right ) ->
  return 0 if left is right
  return -1 if left < right
  1
```

but much shorter and pretty :)