Remove some pointless spec code from validation helpers spec
Support :dataset option to validates_unique vaildation

This is useful any time the model uses a filtered dataset, but
the database has a unique index on the entire table.
Make auto_validations uniqueness validations work correctly for STI subclasses

STI subclasses use a filtered dataset, but the parsed index will be
for an unfiltered dataset, so make sure to use the unfiltered
dataset when setting up the uniqueness validation.
Add update_or_create plugin, for updating an object if it exists, or creating such an object if it does not

This has been requested by a few people recently, and looking
back at GitHub issues, there were a few related to this that
I forgot about, so it seems like a good idea to have this as
one of the officially supported plugins.