Add support for headless testing using Testem

The test suite can now be run by installing testem globally and running
"testem" (or "testem -l phantomjs" if you have phantomjs installed to run tests
headlessly).

If you don't want to (or cannot) install testem globally, you can run
"./node_modules/bin/testem" instead.

These commits:
- Replace inline require shim with a file
- Add testem and configuration
- Add section to the README describing how to run tests
Merge pull request #343 from markprzepiora/testem

Add support for headless testing using Testem