Asset include helpers should consider subdirectories when used with :all

This means that if you have something like:


Those should be included when calling

<%= javascript_include_tag :all %>
<%= stylesheet_link_tag :all %>

I've talked to a couple of coworkers about it and it seems they work
around this by manually specifying the files in subdirectories.

Here's the patch, with tests:

Looks like a no-brainer to me, but please raise your hand if you find drawbacks.