Added ability to opt out uniqueness validation

[VigneshRE]

This is helpful when you want to make the uniqueness validation dynamically decided

[dnagir]

I don't understand why you would put :uniquness => false.

If you want to dynamically decide whether the validation should kick in or not then you can just write
validates :name, :uniqueness => true, :if => :should_be_unique?.

This is how ActiveModel behaves by default.

[dnagir]

I'm strongly against this PR as it is already supported and goes against all the conventions.