Support for new extensions and a reworked greasemonkey icon

Hello there. I wrote [a review](https://addons.mozilla.org/en-US/firefox/addon/adwaita/reviews/412774/) on this excellent theme at addons.mozilla.org some months ago, saying I would share my own additions. I know I am kinda delayed, but it's finally here.

In a nutshell, these commits are style sheets for integrating six new extensions I use (and you may too, because some of them are very popular), as well as proper Gnome-like icons to be used by the css files. I paid attention to sticking to your consensus and respecting the folder structure. I even used optipng and scour to reduce file sizes (very nice tools).

The exact list of added extensions is:
* [flagfox](https://addons.mozilla.org/firefox/addon/flagfox)
* [greasefire](https://addons.mozilla.org/firefox/addon/flagfox/)
* [https everywhere](https://www.eff.org/https-everywhere)
* [tamper data icon redux](https://addons.mozilla.org/firefox/addon/tamper-data-icon-redux/)
* [toomanytabs] (https://addons.mozilla.org/firefox/addon/toomanytabs-saves-your-memory/)
* [WOT](https://addons.mozilla.org/firefox/addon/wot-safe-browsing-tool/) aka Web Of Trust

There's only one modification I performed on the greasemonkey icon, to better match with the rest of your icons and the actual greasemonkey shape. extensions.css had to be changed obviously.

Regarding license terms, I omitted writing a MPL header on every file I created, but take it for granted that I'm releasing all of my contributions under the Mozilla Public License version 2.0 (That's what you are using, isn't it?). In fact most new files are derivatives of yours, so they are already MPL'ed; I just wanted to make it clear because you know the MPL lets you combine FOSS and non-FOSS files in a single work, albeit MPL components remain copylefted. 

Add-ons are a big mess when it comes to aesthetic uniformity since they are so many and so diverse, but whenever I stare at those gnomish Adblock Plus and GreaseMonkey icons, and how well they integrate with the whole theme I realise it's worth making an extra effort. I hope this stuff helps you continue making the best firefox theme out there for gnome-shell/adwaita users.

Best regards.