Adding "call" attribute type

Hi, me again.

I actually have a need to run my own functions to generate factory attributes.

To that end I've suggested here a "call" attribute type, like so:

```
<?php
class ModelA {
  public static $factory = array(
    'slug' => 'call|makeSlug|string'
  );
  public static function makeSlug($string)
  {
    // ...
    return $new_string;
  }
}
```

As you can see, I've also added the ability to pass in other FactoryMuff generated things as a parameter to your static generator function.

The big "gotcha" with that is that if you use a "factory|Model" as the parameter to pass to your callable, you get the instance of the model rather than it's id, as you normally would.

Love to know what you think.

Pete