A Couple New Array Selector Methods

I found them useful in streamlining some of my code.

"nths" corresponds roughly to ruby's "values_at" method.  You can pass in multiple indices, or an array of indices to return a selection: 
```javascript
_.nths(['a', 'b', 'c'], 0,3,4) == _.nths(['a', 'b', 'c'], [0,3,4]) == ['a', 'c', undefined]
```

"binPick" allows indexing via a binary array: 
```javascript
_.binPick(['a', 'b', 'c'], true, false, true, true) == _.binPick(['a', 'b', 'c'], [1,0,'yes', 'truthy']) == ['a', 'c', undefined]
```

Let me know if you find them useful and/or want me to write up some documentation. 

Also, I'm not crazy about the names, so feel free to suggest alternatives.  Thanks!