Definitions path

This allows us to automatically load all .php files in our definitions folder, thus getting rid of the need to define factories in the models themselves. An example would be:

**tests/defintions/User.php**
```php
<?php

use League\FactoryMuffin\Facade\FactoryMuffin;

FactoryMuffin::define('\League\FactoryMuffin\Test\Facade\User', array(
    'name' => 'string',
    'active' => 'boolean',
    'email' => 'email'
));
```

**tests/test/TestCase.php**
```php
<?php

namespace League\FactoryMuffin\Test;

use League\FactoryMuffin\Facade\FactoryMuffin;

class TestCase extends \PHPUnit_Framework_TestCase
{
  public static function setUpBeforeClass()
  {
    FactoryMuffin::setPath(dirname(__DIR__) . '/defintions');
  }
}
```
Could you test it out too?

Just create tests/definitions/anything.php, and try and create a definition.