added .parallel and .sequential method as shortcuts to .then

`parallel` is a shortcut for:

```js
ƒ.then(array.map(function(item) {
  return function(error){}
}))
```

```js
ƒ.parallel(array, function(item, i, error) {})
```

`sequential` is a shortcut for:

```js
array.forEach(function(item) {
  ƒ.then(function(error) {})
})
```

```js
ƒ.sequential(array, function(item, i, error) {})
```