don't assume Tags taxonomy exists or there is only one

Fixes #113
Set parent_id when creating Slider, Featured, and Latest

Since parent_id wasn't set to Tags taxonomy root id,
these Taxons were not appearing in the admin, and
were not being destroyed when the Tags taxonomy was
destroyed.

Fixes #112
spree sample data has 16 products, not 17

Fixes #111
Merge remote-tracking branch 'upstream/2-2-stable' into 2-2-stable