Fix footer line and autoloads

- Fixed footer line for package.el compatibility
- Don't automatically alter eshell hook: this is bad practice
- Update documentation to describe use of hook


See milkypostman/melpa#905

Further, I'd suggest that you remove the `+` from the `+blah-blah+` variable names: it is more idiomatic in elisp to use a consistent prefix for identifiers in a package, any `nyan-prompt-` is fine for that. Occasionally, a double-dash is used to denote private identifiers, e.g. `nyan-prompt--nyan-cat-emoticon` or `nyan-prompt/nyan-cat-emoticon`.