Customization options and validation consistency for validates_uniqueness_of using db constraints

This patch extends #3486 to be consistent with standard AR validations and support customized error message. #3486 patches ActiveRecord to convert db-generated errors for unique constraint violations into active record error messages.

This patch addresses some of those concerns and extends that functionality to:
1. Add hooks through validates_uniqueness_of to support customized error messages

# These two are equivalent functions: lazy option skips standard proactive validation query
validates_uniqueness_of     :name, :message => 'already used!", :lazy => true
handle_unique_key_violation :name, :message => 'already used!"

# Extend the functionality to all AR objects
ActiveRecord::Base.handle_unique_key_violations

1. Keep save! consistent with standard validation functionality
   The current implementation does not add errors for save! and raises RecordNotUnique instead of RecordInvalid exceptions.

• Add error messages to the AR instance as with standard validations. The following functionality would no longer work since no error messages are populated

begin

 record.save!

rescue => e

 flash[:error] = record.errors.full_messages.first
 redirect_to :blah_url

end

• Raise a RecordInvalid subclassed exception for explicitly declared validations. Convenient for rescues_from functionality that looks for RecordInvalid

The patch always adds error messages for save and save!, but only raises RecordInvalid when validates_uniqueness_of is explicitly declared. Conversion of RecordNotUnique to AR errors is exposed in add_record_not_unique_errors to support for further customization (when explicitly handling RecordNotUnique).

The attached patches are based on current (and include #3486)