Implement pick (squashed #135)

Squashed all commits from https://github.com/caolan/highland/pull/135
Should the doc variable assignment not look like this:

```javascript
var docs = [
 {breed: 'chihuahua', name: 'Princess', age: 5},
 {breed: 'labrador', name: 'Rocky', age: 3},
 {breed: 'german-shepherd', name: 'Waffles', age: 9}
];
```