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
the thing is that you'll always call that to_mash... but i get your point.
Actually i think the all problem is that i called it (Fish, then Cash, then Yash :D) but what about i bring the all behaviour to Mash? There will be less surprises since it's just loading files into Mash.
so we'll get this API: 

```ruby
Mash.load('config/settings.yml')
Mash['config/settings.yml']
Twitter.extend Mash.load('config/settings.yml').production # i guess your favorite :D
```

And this would be sexy:
```ruby
Mash.load('config/settings.yml', parser: Extensions::Mash::ErbYmlParser)
Mash.load('data/users.csv', parser: Extensions::Mash::CsvParser)
Mash.load('data/users.txt', parser: CustomParser)
Twitter.extend Mash.load('data/users.txt', parser: CustomParser, mash_method_name: 'data')
Twitter.data => the mash
```

what do you think? @dblock (thx for taking time to give feedback! i also love the way it has evolved!)