sends default behavior in the override block

Here's an idea for issue #70 regarding providing a way to use the default exposure in an override block. This will execute and send the default exposure if the blocks arity is 1, and it doesn't throw a name exception. still needs tests, and probably finer exception handling, but it's a thought. i tried it in a local rails project and it produced the expected results:

```
class ProductsController < ApplicationController
  expose(:products) {|default| default.limit(2) }
  expose(:product)

  def index

  end
end
```

do you like this direction? maybe you can give me some input to improve it?