New configuration option config.plugin_paths which may be a single path like the default 'vendor/plugins' or an array of paths: ['vendor/plugins', 'lib/plugins'].  Plugins are discovered in nested paths, so you can organize your plugins directory as you like.  Refactor load_plugin from load_plugins.  Simplify initializer unit test.  Closes #2757.

git-svn-id: http://svn-commit.rubyonrails.org/rails/trunk@2904 5ecf4fe2-1ee6-0310-87b1-e25e094e27de