Cash: ability to load config files through the magic of hashie :)

Still need to update the changelog and readme... and squash :)

how you guys like it? cc @dblock
I still find the `default_namespace` business too complicated. In a real production application you have many config files and not *all* have the same structure. Worse, if you use Yash in more than one project as a dependency this affects libraries globally, which is probably either very hard or impossible to debug.

It should be possible to write this:

```ruby
Twitter.extend Yash.new('settings/twitter.yml').production
Facebook.extend Yash.new('settings/twitter.yml').production
```