Update zip_up_extension Rake task

Attempt to avoid confusion, by printing the location of the compressed
Chrome Extension.
Add screenshots for the Chrome Web Store

To create the screenshots:

* I used the Window Resizer Chrome Extension[1] to resize the browser
window to 1280 x 800.

* I used Preview.app to resize the screenshots to 1280 x 800 as the
actual images were 1288 x 88.

* I used Glui[2] to add the captions.

[1]: https://chrome.google.com/webstore/detail/window-resizer/kkelicaakdanhinjdeammmilcgefonfh?hl=en
[2]: http://glui.me/