Add test environment.
Use sqlite3 for test and development.
Use 'test' env for tests.
Use bundler for tests.
require main sinatra app for tests
Setup factory_girl.
Fix naming typo (DOH)
Add faker for random email addresses.
Allow a code to have many users.
Begin adding tests for code creation.
Add migration to remove paired_code_id.
Remove Code.create_pair method.
Prevent more than two users from being created.
Replace deprecated find_by_* matchers with AREL style where.
Add Code#assign_user.
Add User#pair & User#has_pair?.
Update sinatra app to use new user -> code relationship.
Remove before_create callback in favor of using super.
Make validation method private.
Add test showing User#ensure_value doesn't override existing value.