Add folders app and config to the engine's gemspec if they're available

railties/lib/rails/generators/rails/plugin_new/templates/%name%.gemspec

@@ -4,6 +4,6 @@
   s.name = "<%= name %>"
   s.summary = "Insert <%= camelized %> summary."
   s.description = "Insert <%= camelized %> description."
-  s.files = Dir["lib/**/*"] + ["MIT-LICENSE", "Rakefile", "README.rdoc"]
+  s.files = Dir["{app,config,lib}/**/*"] + ["MIT-LICENSE", "Rakefile", "README.rdoc"]
   s.version = "0.0.1"
 end