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.

lib/generators/rails/USAGE

@@ -0,0 +1,6 @@
+Description:
+    Generates a serializer for the given resource.
+
+Example:
+    `rails generate serializer Account name created_at`
+