A simple Base implementation for Columbo

Hello Nuno,

I'm submiting to you a "base" implementation for the resources classes "droplet.rb", "image.rb", "region.rb" and "size.rb"

It should clean up lots of the duplicated code removing the "initialize" method from each one of them and giving you some "sugar" methods like "find_by_" a class attribute.

Please notice that for each child class of "base" will need to implement the "self.plural" method, returning the plural of the class name. For instance:

```ruby
class Droplet < Base
  def self.plural
    'droplets'
  end
end
```

### Note

I didn't write any tests for this new methods nor i did test the code so merge it with your code at your own risk.

At lest tell me how do you feel about the "pseudo" improvements.