Add stylesheet support for main.html

This allows to add stylesheets the same way as including javascript source files:

Ex:

```python
class HellowWorldApp:

    stylesheets = ['http://code.jquery.com/mobile/1.4.0/jquery.mobile-1.4.0.min.css']

    include = ['http://code.jquery.com/jquery-1.10.2.min.js',
               'http://code.jquery.com/mobile/1.4.0/jquery.mobile-1.4.0.min.js']
```