Add support for chained dependencies

## What this is

This pull request add support for chained dependencies. e.g.

```php
array(
  'class' => Phactory::has_one('Class'),
  'dep1' => Phactory::uses('class.value'),
  'dep2' => Phactory::uses('dep1.value'),
);
```

I also added an exception message to `Dependency` to make it easier to debug broken dependencies.