Add package dependencies, plus other minor clean-up

- Fix typos in boilerplate (please consider using auto-insert-mode)
- Depend on Emacs 24 (due to lexical-binding)
- Depend on cl-lib 0.2, for Emacs 24.2 and below
- Fix checkdoc warnings in docstrings
- Don't use auto-mode-alist for special buffers: it's not idiomatic