Helpers for recipe authors

- Add helper function `package-build-create-recipe`
- Add `package-build-minor-mode`, enabled for recipes: C-c C-c builds current recipe
- Opening recipe files causes `package-build` to get loaded, and
  `package-build-minor-mode` to get enabled
Merge pull request #607 from milkypostman/create-recipe-helpers

Helpers for recipe authors