Handle `dev:prime` seeds in convetional file

Rails' convention is to do DB seeding in `db/seeds.rb`. This moves the logic
from the `.rake` file to the `.rb` file.

It has the nice side-effect of being called via `rake db:seed`, in addition to
`rake dev:prime`.

Also, is allows for calls to `FactoryGirl` to be fully out-dented and not
crammed inside of nested blocks.
You can define a module method in this file, extend `FactoryGirl::Syntax::Methods`, and invoke the method at the end:

``` ruby
module Seeds
  extend FactoryGirl::Syntax::Methods

  def self.run
    create(:user)
    # ...
  end
end

Seeds.run
```

That gives it some structure and resolves any confusion about scope.