chore: source maps for Karma/Gulp

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

karma-dart.conf.js

@@ -57,6 +57,7 @@ module.exports = function(config) {
     traceurPreprocessor: {
       options: {
         outputLanguage: 'dart',
+        sourceMaps: true,
         script: false,
         modules: 'register',
         types: true,