support sequel's STI model_map functionality
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.

rpanachi/fixture_dependencies,brianphillips/fixture_dependencies,kematzy/fixture_dependencies,brianphillips/fixture_dependencies,rpanachi/fixture_dependencies,jeremyevans/fixture_dependencies