Added permissive Parameter.hash constraint

This PR allows you to just specify a Hash constraint while keeping the keys/values of the hash open.

For example:

````ruby
param.permit(
  id: Paramter.id,
  url: Parameter.string,
  metrics: Parameter.hash
)
````

Would allow:
````json
{
  "id": 123, 
  "url": "http://google.com/",
  "metrics": {
    "user": "john_doe",
    "timeOnPage": 124,
    "clicks": 13 
  }
}
````