refacto(test): karmify the tests

Hi @guybedford 

Following #250 
Here is a _karma-fication_ of the tests. [Travis result here](https://travis-ci.org/douglasduteil/es6-module-loader/builds/45437849)

- `npm test` : run the tests without the polyfill
- `npm test:polyfill` : run the tests with the polyfill
- `npm test:perf` : run the benchmarks

Notes: 
- A lot of `karma-*.conf.js` files just to keep things simple. A common script can be made in the future.
- The `script[type="module"]` test is still unsolved. Don't really know how to _karmify_ it properly. Suggestions are welcome.
- The benchmarks results aren't fixed. I means it's normal depending of where it's running, but really Travis has some pretty low results... Tell me if it's an issue for you. We can just remove the benchmarks runner on it if you want.

Closes #250