use this instead of window and check for jquery

Hi,

I'm not used to lisp, but liked the idea and would like to use it in my rails project (even though I know it actually makes no sense...)
So I write a really small gem to enable it within Rails asset pipeline (https://github.com/tadeuzagallo/sibilant-rails), hope you don't mind...
But while developing, I needed to compile the browser version with ExecJS, and the browser version was incompatible because of the use of `window` and the hard dependency on `jQuery`.
I changed the `window` for `this`, and made the `jQuery` part of the script optional.
I thought it would make no harm to the master branch, if you would like to merge.

Congrats for the nice project.