Patch: Load metal from Engines

Here's a ticket regarding loading /app/metal from inside plugins
http://rails.lighthouseapp.com/projects/8994-ruby-on-rails/tickets/2045-patch-metals-should-be-loaded-from-engines

Take a look,
let me know what you think.

I think I have a solution that works.

Maybe it doesn't matter,
but it seems odd that Engines can do everything apart from Metal at
the moment.