wip: bundles refactoring

Usage of the bundles:

```jinja
{% bundle "10-jquery.js", "50-invenio.css" %}
```

Definition of the bundles:
```python
# invenio.modules.mymodule.bundles
from invenio.ext.assets import Bundle

js = Bundle("js/query.js", output="gen/jquery.js", filters="uglifyjs", name="10-jquery.js")
css = Bundle("less/invenio.less", output="gen/invenio.css", filters="less,cleancss", name="50-invenio.css") 
```

The bundles are collected via Flask-Registry.

**TODO**
- [ ] `inveniomanage assets build` compiles the bundles
- [ ] drop the old `js`/`css` syntax