Lots of refactorings and a few minor improvements.

Lots of refactorings and a few minor improvements.

User visible improvements and changes:
* Improve and debug `rst-forward-section` and `rst-backward-section`.
* Auto-enumeration may be used with all styles for list insertion.
* Improve and debug `rst-toc-insert`.
* Adapt change in Emacs to use customization group `text` instead of `wp`.
* Bind `n` and `p` in `rst-toc-mode`.
* `z` in `toc-mode` returns to the previous window configuration.
* Require Emacs version >= 24.1.

Lots of refactorings including:
* Silence byte compiler.
* Use lexical binding.
* Use `cl-lib`.
* Add tests and raise test coverage.