Modify haxe.Int64.hx to work on php and fix other issues

1) On the php platform, integer overflow and ushr are incorrect (for 32-bits Int), this made the 64-bit Int emulation wrong. Special functions now correct for these faults in the Int64 class.
2) Both div and mod should now have the sign correct when double-negative, but a Haxe guru should check the new mod sign logic is as required by Haxe, see http://en.wikipedia.org/wiki/Modulo_operation for the choices available.
3) Special cases of div or mod by 0 or 1 should now be correctly handled, including throwing an error on divide by zero, but again a Haxe guru should check if that is the correct thing to do for Haxe.