Added _.reverseOrder

This is a non-mutating version of `Array.prototype.reverse`. It is named differently so that people won't be confused by the fact that when using Underscore's chaining syntax, `reverse` is a mutating method. I went back and forth on the name a little bit, but this is the best I could come up with. Just let me know if you have a better idea.

It will throw a `TypeError` if you try to use it on a string, because it can't properly handle reversing unicode character sequences. See [Esrever](https://github.com/mathiasbynens/esrever) for more details.