Switches headless browser from PhantomJS to Nightmare

[fabrik42]

I'm thinking about switching the headless browser lib from PhantomJS to Nightmare.

There are some Pros and Cons:

Pro:

• Nightmare uses Electron, which has much better support for new browser features, e.g. WebGL (we could render screenshots in much more interesting ways).
• PhantomJS still is buggy sometimes, e.g. when setting the viewport.
• I've never encountered any SSL Problems with Nightmare, with PhantomJS very often (see readme).
• I hope for better cross platform compatibility.

Con:

• More dependencies, need to do an npm install
• Nightmare docs are as bad as the PhantomJS docs. :( (got help in the GH issues)
• Still need to figure out, how to make better use of the ES generators feature in this project.

Feedback is very welcome!

Best,

Christian