Validates associated uniqueness for nested attributes

As pointed out in #1572 and #4568, there is no uniqueness validation for nested attributes. It does check against the db for duplicate records but doesn't check the collection being passed in. This checks for that only when there is a uniqueness validation defined in the associated model.

It is a basic uniqueness validation by combining the attributes defined in the uniqueness validator. I've chosen to only pull the necessary attributes to generate the "key" instead of all attributes https://github.com/jeyb/rails/commit/1f618ff24e284e6aad15da43952ed3d45cc1d621 in case there are timestamps that or other user generated content that throws off the uniqueness check.

Let me know what you guys think.