Document method for determining whether a moment() is valid

[wolever]

Currently there doesn't seem to be any straight forward method of determining whether a moment() is valid or not (ex, because a parse failed).

Various methods (ex, isNaN(+moment("asdf"))) work, but it would be nice if there was a documented right way to do this.

I would propose:

moment.fn.valid = function() {
    return isNaN(+this);
}

But there is no doubt a better way.

[timrwood]

Check out the discussion on #235