For Karma, the source mapa are inlined inside each source file.
For `build/*` output, separate `*.map` file is created.
This changes the API of `tools/transpiler/index.js`. I believe this API
is only used in `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
Vojta Jina
Tobias Bosch