Add .eslintrc, .eslintignore and fix most reported errors.

I am a bit surprised
`"use strict";`
is not recommended in the
https://github.com/mozilla/addon-sdk/wiki/Coding-style-guide
or
https://developer.mozilla.org/en-US/Add-ons/SDK/Guides
given all the other advanced stuff being advertized (arrow functions, let, etc.).

So I left that out of my changes but I think it would be good to add a global
`"use strict";`
to the examples wherever possible.

My single to double quote conversion needs attention, but I made sure to use eslint's `avoid-escape`
`"quotes": [ 2, "double", "avoid-escape" ]`
when checking.

Found a few missing semicolons and added them.