Simulate meteor's "load files in subdirectories before files in parent directories" load order rule.

Meteor's load order loads files in subdirectories before files in parent directories. This PR changes rtd to do the same. 

They way I've done it is a bit clunky however, so perhaps someone more experienced with grunt & karma may be able to suggest a better approach:
- it will only deal with directory structures a couple of layers deep;
- I couldn't figure out how to get it to respect the other meteor load order rule, where "files that match main.* are loaded after everything else".