Make beeminder.el feel more like other Emacs Lisp packages

- Remove redefinitions of variables in other packages; require the
  appropriate packages instead
- Remove beeminder-scratch and use let when needed. (Lots of functions changed)
- beeminder-on-org-task: Check for done-ness using the Org way to do things
Make beeminder-on-org-task-completed non-interactive
Merge pull request #8 from sachac/make-it-more-elispy

Make it more elispy