Update README + Add simple dev server for running locally

[nmielnik]

Q	A
Bug fix?	no
New feature?	no
BC breaks?	no
Deprecations?	no
New tests added?	not needed
Fixed tickets
License	MIT

Description

Re-arranged README to:

• Bring basic screenshot above the fold
• Link to the actual demo via the screenshot
• Make the basic installation steps easier to find
• Update the image/link to saucelabs
• Add documentation for quickly running medium-editor demo

I've always had a basic static server file for running/developing/testing medium-editor locally, figured I should probably just check it in and save new people a little big of time when trying to run things locally. I checked it in as index.js which can just be run via node index.js after doing an npm install.

[coveralls]

Coverage remained the same at 94.743% when pulling 49a0bd7 on basic-updates into 9b511b2 on master.