Add tab switcher shortcut addon

[callahad]

Add-on for the the browser that puts a switch-tabs button in the top bar.

[callahad]

r? @mikehenrty

[mikehenrty]

Awesome Dan! Thanks for submitting this.

Your manifest at https://callahad.github.io/fxos-tabswitcher-addon/manifest.webapp is missing the package_path property. Without this, hackerplace won't be able to install your app. You need to zip up your js file and icon, and then put that zip at the package path. See https://github.com/mikehenrty/fxos-cracked/tree/gh-pages.

[mikehenrty]

@callahad Do you think we should add this step to the README of hackerplace?

[callahad]

Hah, whoops. Pushes fine from WebIDE. Fixing now. A reminder in the README would be handy.

[callahad]

Added a package_path and force pushed to my branch. r? @mikehenrty

[callahad]

Oh, I'm a doofus. Fixing one typo and re-pushing real quick.