Enable sprig seed file generation (yaml only at this point) from database records (addresses #3)
Allow users to specify environment and classes when calling Sprig::Harvest.reap (addresses #3)

- Created a Sprig::Harvest class-level configuration
- `Sprig::Harvest.reap` takes an options hash and sets environment (`:env` key) and classes
  (`:classes` key) on Sprig::Harvest's configuration
Extract TsortableHash into its own file due to re-use in multiple places
Extract Sprig::Harvest's configuration into its own file

- Enable Sprig::Harvest to handle arguments from the command line (passing ENV via Rake task)
Add `db:seed:reap` rake task (fixes #3)
Update README with seed file creation from database
Use `=>` hash syntax throughout for Ruby 1.9.3 support
Replace eigenclass method with `singleton_class`
Modify #generate_sprig_id so that it accounts for existing non-integer sprig_ids
Remove unnecessary user_id fields
Modify Sprig::Harvest.reap to work with STI and module-nested models
Change `reap` option from "classes" to "models"
Extract tsort logic out of `.all` method to simplify it
Refactor the way a Harvest::Record's underlying AR model methods are wrapped using method_missing

Thanks @ltk