Added permissive Parameters.map constraint

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