Add global callbacks

This allows callbacks (after :build, :create, etc.) to be defined at the
FactoryGirl level; this means that the callback will be invoked for all
factories. This is primarily to maintain consistency and follow the
principle of least surprise.

As usual, callbacks are applied from the lowest component to the
highest, meaning that global callbacks will be run after factory and
trait callbacks are run.

    FactoryGirl.define do
      after(:build) {|object| puts "Built #{object}" }

      factory :user
      # ...
    end

Closes #481
Closes #486