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 removed the namespace as an init option, though the idea of default namespace is for then you'll want to load all your config files with a default namespace. 
ie: when you'll want to load all your config files in the same environment.

```yml
#twitter.yml
development:
   api_key: 'bar'
production:
   api_key: <%=ENV['FOO'] %>
```
```yml
#github.yml
development:
   api_key: 'bar'
production:
   api_key: <%=ENV['FOO'] %>
```
Now, we can define a default namespace like this:

```ruby
Yash.default_namespace = 'production' # or RACK_ENV or whatever

yash= Yash.new('config/settings/')
yash['github.yml'].api_key # => production api_key from github.yml
yash['twitter.yml'].api_key # => production api_key from twitter.yml

```
without default_namespace, you can still access them like this:

```ruby
yash= Yash.new('config/settings/')['twitter.yml'].production.api_key
```
what do you think?