Added _.unzip as inverse to _.pairs and _.zip

Very often I've found the need to take an object (often nested) and convert it to an array of pairs via `_.pairs` to perform some sort of tree-walking or sequential operation.  Likewise, I've also found many cases up using `_.zip` to create an array of pairs for processing and eventual object creation via `_.object`.  To support these types of operations I've built a large tool set for processing pairs, sequences and alists (arrays of pairs).  However, there is a link missing in the beautiful symmetry created by `_.zip`, `_.pairs` and `_.object` and that's `_.unzip`.  There is a ticket for something called "unzip" at #41, but it operates differently than I propose.  That is, `_.unzip` is meant to allow one to come full circle from a pair of arrays, to an array of pairs, to an object, back to an array of pairs and finally back to a pair of arrays.  It's action is as follows:

    var names = ['moe', 'larry', 'curly'], ages = [30, 40, 50];
    var zStooges = _.zip (names, ages);              // pairs of stooge data                                              
    
    _.unzip (                                        // put back together                                                 
      _.pairs (                                      // back to pairs                                                     
        _.invert (                                   // swap keys/values                                                  
          _.object (                                 // objectify                                                         
            _.map (zStooges, function (pair) {       // convert the head of the pair to upcase                            
      return [pair [0].toUpperCase (), pair [1]];
    })))));

Now obviously you'd write this code more beautifully than this, but the example is meant to show that a data structure comes full-circle[^1].  I've found `unzip` highly useful and think it would complete a beautiful relationship between highly useful functions.

*Apologies for the duplicate of #1015. I've been working in Jira for so long that I wasn't prepared for a ticket system that worked well.*

[^1]: It almost comes full circle but the call to `_.pairs` for some reason converts the `Object` number keys to strings. :-(  That's a different matter outside the scope of this ticket.