Add the :allow_blank option to validations of AWS::Record that can support it.

This PR adds the :allow_blank option to a number of the validators in `AWS::Record`.  This allows for better compatibility with libraries that expect the validations provided by `ActiveModel` and allows for `AWS::Record` to be more of a ready drop in replacement for `ActiveRecord`.  

For instance Devise expects to be able to pass `:allow_blank => true` to `validates_uniqueness_of` and `validates_length_of`.  Without that option it is only possible to update a `User` record by resetting the `password` and the `password_confirmation` each time.