Add better error checking for app IDs as int when greater than PHP_INT_MAX

[SammyK]

Fixes #532

Facebook app ID's are really big and on 32-bit systems many are bigger than PHP_INT_MAX causing integer overflow issues. We can't fix when people overflow an int:

$appId = PHP_INT_MAX + 10;

But we can at least give them better error messages and prevent overflows when FacebookApp is unserialized. :)

[SammyK]

@gfosco This bug fix is also ready to go. :)