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
Definitely agree that the default constructor should take the filename as the first argument and then a collection of options, which may support `namespace`. For simplicity-sake I would kill `namespace` though and just do:

``` ruby
database_config = Cash.new('config/database.yml').production
```