Allow to use preload helpers inside factory definition

In the application that you are testing add this line to your test helper file:

```ruby
require 'factory_girl/preload'
require 'factory_girl/preload/factory_girl'
```

Now you can use the helpers outside a preload block:

```ruby
FactoryGirl.define do
  factory :projects do
    user { users(:john) }
  end
end
```