chore: source maps for Karma/Gulp

This changes the API of `tools/transpiler/index.js`. I believe it’s 
not used anywhere besides `gulp-traceur.js` and
`karma-traceur-preprocessor.js`. Instead of returning the transpiled
string, `compile()` returns a result object such as:
```js
{
  js: ‘transpiled code’,
  sourceMap: null || {}
}
```

Closes #20
I could understand why we bundle the source maps with original code for running tests, but why not have separate files for the build artifacts?