baseUrl option

Consider the following layout:
```
/project_root
|-- gulpfile.js
|-- resources/
|---- public/
|------ deps.js
|------ module.js
```

And server is set up to serve public folder as `/`. After running gulp-closure-deps we'll get `../../../resources/public/module.js` path to dependency in `deps.js`, and it won't be found.

`baseUrl` option i've implemented lets you set it to `"resources/public"` and all deps will be written relatively to that url.