New widget positions and templates

Aritz, thank you very much for the awesome theme! Here is a suggestion with some changes I needed when using the theme. PM me if you want to see a working example. Thanks again!

Changelog:

- Added widgets: Several new widget positions, some with Bootstrap
Jumbotron. See screenshot_widget_positions.gif
- Added templates: Frontpage and About us. Use these with widgets to
achieve greater variations between pages of your site
- Added css: Couple classes to markup for handy hooks such as "main"
column, and a couple generic css rules