Only load generators when needed

- use hook_for to hook in the serializer and remove load_generators
- move generators so they can be found by rails
- move to_prepare block to railtie config

This commit improves the way the generators are loaded and how
they extend the resource generator.

* The initializer block has been changed to a `generator` block which is only executed when generators are needed.
* The call to `app.load_generators` has been removed. There is no need to load *all* generators.
* The `resource_override.rb` has been changed to use `hook_for` to extend the resource generator.
* The directory for the generators has been moved to match the way Rails looks to load generators.

With `hook_for` it would now be possible for a user to pass `--no-serializer` to skip that option.
The `--serialize` option also now shows up in the generator help with `rails g resource --help`.

These changes follow the way the Draper gem extends the `controller` generator.

test/generators/scaffold_controller_generator_test.rb

@@ -1,4 +1,5 @@
 require 'test_helper'
+require 'generators/rails/resource_override'
 
 class ResourceGeneratorTest < Rails::Generators::TestCase
   destination File.expand_path('../../../tmp/generators', __FILE__)

test/generators/serializer_generator_test.rb

@@ -1,5 +1,6 @@
 require 'test_helper'
-require 'generators/serializer/serializer_generator'
+require 'generators/rails/resource_override'
+require 'generators/rails/serializer_generator'
 
 class SerializerGeneratorTest < Rails::Generators::TestCase
   destination File.expand_path('../../../tmp/generators', __FILE__)