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
Add docs to help users upgrade from < 1.0, fixes #877

* Help with new command line usage
* Help with deprecated flags
* Help with baseurl
* Help with drafts
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.
Hard line wrap FTW!
s/problematically/programmatically/, props @mojombo
De-deprecate `--safe`, props @mojombo
Note that `--auto` is not `--watch`
clean up language a bit to clarify meaning
Add draft functionality explanation to upgrading docs
First pass at baseurl documentation
s/root/source/, props @parkr
fix typo in draft post explanation, props @parkr
Direct users to the proper command when previewing drafts.
clarify baseurl usage
quick shout out to jekyll new
86'd Chrome Frame in template site
Make --config usage more explicit.

See https://github.com/mojombo/jekyll/pull/965#discussion_r3797116 and https://github.com/mojombo/jekyll/pull/965#discussion_r3797085

Props @parkr.
Clarify new way to do server and auto, props @parkr

see https://github.com/mojombo/jekyll/pull/965#issuecomment-16396829
plurlize file(s)
86'd conditional IE classes