Add support for headless testing using Testem

These commits are a tiny first step towards merging my ES6 modules branch.

They add support for running the Ramda.js test suite using the Testem test runner, which allows tests to be run across different browsers with results shown in your terminal, and live-reloaded as any changes are made to production code or test code.
This is the only extension currently tested in `test/index.html` so I wasn't sure whether to include others in the testem config.