Allow pick to return multiple elements

Similar to jashkenas/underscore#963 and Ruby's [sample](http://www.ruby-doc.org/core-1.9.3/Array.html#method-i-sample), allow `pick` to take a count argument that will return that many random elements.


```javascript
var suits = ['spades', 'hearts', 'clubs', 'diamonds'];
var deck = _.times(52, function(n) {
  return { value: n % 13, suit: suits[Math.floor(n / 13)] }; 
});

var hand  = chance.pick(deck, 7);
```