support sequel's STI model_map functionality

Previously, fixture-dependencies assumed the :sti_key value was the
class name.  This patch delegates the loading of the sub-class to the
:sti_load method in the STI plugin class.
Add specs for Sequel single table inheritance
Bump copyright year
Make sure spec database is migrated before running specs

Use same ruby interpreter to run sequel as is running rake.
Bump version to 1.3.2, add license and homepage