Add a before_create callback

Closes #323

lib/factory_girl.rb

@@ -64,6 +64,6 @@ module FactoryGirl
   end
 
   def self.callback_names
-    [:after_build, :after_create, :after_stub].freeze
+    [:after_build, :after_create, :after_stub, :before_create].freeze
   end
 end

lib/factory_girl/strategy/create.rb

@@ -8,6 +8,7 @@ module FactoryGirl
       def result(attribute_assigner, to_create)
         attribute_assigner.object.tap do |result_instance|
           run_callbacks(:after_build, result_instance)
+          run_callbacks(:before_create, result_instance)
           to_create[result_instance]
           run_callbacks(:after_create, result_instance)
         end

spec/factory_girl/strategy/create_spec.rb

@@ -2,7 +2,7 @@ require 'spec_helper'
 
 describe FactoryGirl::Strategy::Create do
   it_should_behave_like "strategy with association support", FactoryGirl::Strategy::Create
-  it_should_behave_like "strategy with callbacks", :after_build, :after_create
+  it_should_behave_like "strategy with callbacks", :after_build, :before_create, :after_create
 
   it "runs a custom create block" do
     block_run = false