xecmascript-ava

First few problems from Exercism's ecmascript track converted to use the ava test runner.

This has following advantages:

• Remove use of any build tool to run tests
• Modern testing env with nice diffs

Related issue here: exercism/javascript#272

Steps to run tests:

• cd into any exercise directory.
• For the first time, you will have to install all required dependencies using npm install.
• Run the tests using npm test.