[Radiant] Extension load order

[Daniel Sheppard]

Rails has just changed their mechanism for plugin loading:

http://ryandaigle.com/articles/2007/9/24/what-s-new-in-edge-rails-specify-plugin-load-order

Basically, you specify the load order of plugins that you care about and then just add the
:all symbol to substitute in all the remaining plugins.

I'm going to put in the same mechanism for loading extensions. I've got most of the code 
done, just need to fix up a few tests. Probably check it in in the next day or so.

Dan.