Various fixes

* Remove activation lexical-binding, which is not used, and would limit the package to Emacs 24 only
* Fix doc formatting, recommend require instead of load
* Remove malformatted Package-Requires line
* Autoload insert-shebang function
* Don't modify find-file-hook directly: this is bad practice
* Fix custom group
Merge pull request #1 from purcell/patch-1

Various fixes