Misc fixes

- Remove lexical-binding: I don't think it's required, and it implies that this code can't work with Emacs 23, which I'm not sure is the case
- Don't depend on `cl`, which isn't an installable package
- Depend on specific versions of `elfeed` and `org`
- Fix docstrings to keep `M-x checkdoc` happy
- Fix some code formatting
- Prefer `mapc` to `mapcar` when the results are discarded
- Autoload the setup function, and let the user call it, rather than calling it when the lib. is `require`d. This is more idiomatic.

In connection with https://github.com/milkypostman/melpa/pull/2056 :-)