Added Font-Awesome support, improved class_fmt()

- added bower dependency: font-awesome
- updated 'grunt dist' task to copy the fonts into static/fonts
- updated vendor.html stylesheet
- 'sprintf' filter now accepts variable number of arguments
- added $icon value type to bootstrap.py
- added icon template to bootstrap.yml
- class_fmt macro is renamed to div_class_fmt
- added class_fmt context function which does the heavy lifting
(arguments are now passed as **kwargs)
- updated example.py