run the unit tests through babel

[ColinEberhardt]

Currently the unit tests are run against the transpiled code in the lib folder. This has a few issues:

1. it increases the build / test time
2. the test code can only use language features currently supported node
3. the import statements in test code reference the transpiled output, which makes it hard to navigate failure stack traces.
4. We don't have any flow-typing for the unit test code

Ideally mocha would be run through babel - with source maps.