Tweaks to force consistent development

I was going to start figuring out the toolbarbutton bug (d747da didn't fix it for me either, nor did the updated toolbarbutton.js that I sent you.) In the course of starting work on it, I wanted to make sure that my development environment is the same as yours.

Changes included in this PR:
* Including a LICENSE file that matches the license you reference on AMO
* Including a top-level package.json so node dependencies are met (namely cli-color and commander)
* Enforcing the assumption that addon-sdk is in the top-level directory, and a .gitignore to make sure it never makes it back into the repo (I wanted to have ``npm install`` acquire this, but it quickly grew out of scope to accomplish that)
* Relying on the "path" package in "install.js" for cross-platform friendliness

I hope these changes don't insult your development style. I just want to make sure that I dance like you dance.