Webdriver.io: Implement and refactor base client to more abstract.

[ryan-roemer]

Target library:

• http://webdriver.io/
• https://github.com/webdriverio/webdriverio

POC for a full port: https://github.com/FormidableLabs/rowdy/compare/chore-rowdio-bootstrap

Current work:

• Add note / update package.json with permissive dependencies.
• Globally search + replace in docs on: WD.js, wd
• Add saucelabs status update to all implementations.
• Add webdriverio to Travis.
• Wrap config.js so that it permissively swaps WD.js/WebdriverIO differences (like host for BS/SL, etc.)
• Parallel test execution example for webdriverio.
• Document full examples for wd, webdriverio.

Done:

• Consider an adapter.refreshClient(cb) as an alternative to perTest.
• Abstract out to multiple wrapper instances of clients, away from singleton approach.
• Implement wrapper for WD.js + test examples.
• Implement wrapper for WebdriverIO + test examples.