Various fixes

I saw this library pop up on Marmalade, and I plan to add it to [MELPA](http://melpa.milkbox.net/).

This commit provides the following 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

Really, though, you should provide a `defcustom` which is an alist from major mode to interpreter name. The hard-coded hash table is a bit of a hack which limits the usefulness of the code.

Cheers!

-Steve