Add :within option for numericality validator

The following patch adds the :within option to validates_numericality_of in ActiveModel.

This allows the developer to use:

```ruby
validates_numericality_of :age, within: 18..25
```

instead of 

```ruby
validates_numericality_of :age, greater_than_or_equal_to: 18, less_than_or_equal_to: 25
```