add #isJSON

not sure if there are any objections to adding isJSON, but i added this because it would be useful in this case:
```
if (!validator.isJSON(str)) return false;
// we can continue from this point on without needing try...catch blocks
var obj = JSON.parse(str);
```

of course this will only work when the JSON property exists, which is 98.5% of modern situations. the other 1.5% of the time, an error would get thrown as usual.