Create Feature model, add Directory#features relation.

- Update specs for Directory::has_tree relations.

Current spec status: 51 examples, 0 failures in 0.43065 seconds.
Implement Directory::feature to define feature types.

Mix in FactoryGirl syntax to RSpec example groups for terser specs.

Current spec status: 76 examples, 0 failures in 0.67876 seconds.
Merge pull request #6 from sleepingkingstudios/feature-features

Add polymorphic Feature model.