Porting extension to Firefox

[wistcc]

Note: serve this issue to explain the porting process to Firefox.

Since Firefox doesn't fully support the extension API, I've created 2 PRs, one for Firefox and another for Nightly (because Nightly does support more functions from the extension API).

If we see carefully both PRs, we can notice that the only difference (so far) is that Firefox does not support chrome.storage.async but Nightly does.

Please let me know any suggestions you could have to improve this changes! :)

Resources:

• https://developer.mozilla.org/en-US/Add-ons/WebExtensions
• http://www.arewewebextensionsyet.com/
• https://hacks.mozilla.org/2015/10/porting-chrome-extensions-to-firefox-with-webextensions/