wrapCallback is uncurryable

```
var foo = _.wrapCallback(function (thing, done){
done(null, thing);
});

foo('bar').apply(_.log); // bar

_.curry(foo)('bar').apply(_.log);
// ^
// TypeError: object is not a function
```

The issue is that wrapCallback handles its arguments dynamically. I'm not sure there's any way around it, but I wanted to raise it for discussion as it would be a useful thing to be able to do. I suppose _.partial could be used instead, but that's less elegant.

```
// _.wrapCallback
function () {
var self = this;
var args = slice.call(arguments);
return _(function (push) {
var cb = function (err, x) {
if (err) {
push(err);
}
else {
push(null, x);
}
push(null, nil);
};
f.apply(self, args.concat([cb]));
});
}
```