Remove class indirection.
Rename Perform module to DumpMethods.
Tweak README.
Set `create!` as the default method.

Replace `create` with `create!` as the default method.  We want to know
when our seed data fails to load.
Add support for Rails Engines.

Replace directory-based model loading with Rails class loader.  This
adds support for Rails Engines (and any other models defined in gems)
and also simplifies Seed Dump.
Remove PG_SCHEMA option.
Extract spec helpers into their own file.