Make template site HTML5

It would really pain me if the default Jekyll site were XHTML. That'd be bad for the internet.

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. 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
Strip CSS, JS, and IMG from template site

Per #964, just upgrade the template site from XHTML to HTML5,
leaving the existing CSS. No need to get fancy.

The HTML is essentially HTML5 Boilerplate. No need for a holy war.

Look and feel should remain the same.
86'd Chrome Frame in template site
86'd conditional IE classes
Move site title to H1 tag, wrap header in header tag.
CSS reorganization

1. Fix regression on s/.footer/footer/
2. Fix whitespace and tabbing
3. Reorganize properties more logically
Downgrade default site template for Internet exploder

Swap `<header>` and `<footer>` tags to `<div>`s, so we can support IE without the need for an html5 shim.
Merge pull request #964 from benbalter/html-template-site

Make Template site HTML5