Add nested validator to validate nested children

The goal of this validator is to validate each element inside of the array.

It enable to validate this kind of parameters:
```ruby
{ 
  user_id: 5,
  comments: [
   {
     name: 'name of comment1'
     comment: 'blabla1'
   },
  {
     name: 'name of comment2'
     comment: 'blabla2'
  }
  ]
}
```

The definition of the parameters will be:

```ruby
param :user_id, Integer
param :comments, :nested do
    param :comment, String
end
```

I think this validator could be useful for many people and so should be included by default in the gem.

Any remarks ?