[POST-TAG] Remove install.js (bugs 888360, 891557)

Reasons this is a good thing:

1. Less code, even with all the added comments
2. All install/purchase logic is in one place. One source of truth.
3. Code is written in the order that it executes (!)
4. Everything is deferreds, so no more [playing the argument silly game](http://www.youtube.com/watch?v=x4o-TeMHys0#t=21s).
5. When you're debugging buttons or install, your stack makes sense (it doesn't terminate at the event emitter).
6. Everything in the lexical scope means no more passing junk for no reason.
7. No more `app_install_success` dance for changing the button to Launch. Just call `buttons.buttonInstalled()` with the basic data.

Don't merge until after the tag or @krupa will explode.