Jeremy/validates uniqueness of

Added a validation for uniqueness.  This allows the following functionality.

````ruby
    class Book < AWS::Record::Model
      string_attr :title
      validates_uniqueness_of :title
    end

    book1 = Book.new(:title => "The big book of tests")
    book1.valid? # => true
    book1.save

    book2 = Book.new(:title => "The big book of tests")
    book2.valid? # => false
````