additional support for multi-app projects

My Padrino project consolidates its (currently) two apps in an "apps" directory in the root of the project. padrino-assets only looks at project/app/assets for assets paths, so my apps get missed. I have made it so that any app for which padrino-assets is registered, it automatically adds approot/assets to the load paths.

I also updated the route that serves assets to take into account an app's configured assets_prefix instead of a hard-coded /assets. (That chunk of code is a temporary fix for environments where assets are not precompiled, but what can be done to improve it?)

Lastly I added project/vendor/assets as a default load path. It's a Rails-y convention that I use whenever I have third-party JavaScript or CSS in my app. Shouldn't hurt anything either way.

I didn't break any specs. (But I didn't add any either.)