Merge Chrome and Firefox implementations

[SimenB]

I uploaded the resulting zipfile for Chrome to Mozilla's online check for compat (https://www.extensiontest.com/).

So this PR ports all of the build from gulp to webpack, and deletes all of the firefox code.

~I still want to add add https://github.com/mozilla/webextension-polyfill which allows us to not use any chrome APIs directly. We can also simplify much of the code which is written to abstract over the browser differences. The polyfill also gives us Promise APIs, which another chunk of the code in this repo also does.~ Done

[SimenB]

@adriancole @eirslett This can be reviewed & (hopefully) merged. I'd like to add some tests before release, but I'm also gonna do a small reorganizing of the files, and then git thinks it's all new files. So (squash) merging this separately makes for a more accurate history 🙂

[codefromthecrypt]

fyi "npm run dev-chrome" or firefox doesn't work (might be unrelated)

[SimenB]

It's just npm run dev now, no difference between chrome and firefox 🙂

[codefromthecrypt]

was able to test chrome via your instructions

However, FF doesn't seem to like the build directory (when I go to install add-on from file)

[codefromthecrypt]

decided on gitter to handle FF separately