Make Template site HTML5

It would really pain me if the default Jekyll site were XHTML with some pretty deprecated practices. That'd be bad for the internet if lots of people were to do it.

Instead, use Twitter Bootstrap as a base (while still keeping @mojombo's origin design), to give users a great initial baseline to build beautiful, simple sites.

Defaults are more than just the starting point. It's what 80% of users are going to use. Look at WordPress and the default theme. Countless child themes are based off of it because users look to it as the best of the best. May as well lead by example and start the user off right.

* Add Bootstrap base CSS as a better reset and base layer
* Update default layout to HTML5 with boilerplate best practices
* Add title and post date to post.html (rather than including in the post itself)
* Make site title a variable and add to _config.yml
* Add page title to header
* Add default .gitignore to ignore `_site`
* Remove unused `rss.png` and `.gitkeep`
* Add Modernizr for legacy IE support