Update the site for the 1.4.0 release

Add a release announcement and update the History file
bump the version number in the code
prep the gemspec for 1.4.0
Use YYYY-MM-DD in History file :)
1.4.0 release post fixes
Merge pull request #1785 from mojombo/release-1.4.0-updates
Release 1.4.0
Merge pull request #1657 from mojombo/safe-whitelist

A gem-based plugin whitelist for `safe` mode
Update history to reflect merge of #1657
Don't check for >= 1.9 in Rakefile
Remove Ruby <1.9 core extensions
Remove checks for Ruby 1.8.x
Add tests in all the time that was omitted for Ruby 1.8.x
Remove File.read_with_options patch to allow 1.9 and 1.8 to behave the same
Move CSS files to includes.
Upgrade to normalize.css v2.1.3
Merge pull request #1787 from mojombo/css-includes-2

Move jekyllrb.com CSS to includes & Update Normalize.css to v2.1.3
Update history to reflect merge of #1787
Implement group_by Liquid filter & tests.
Mention the 'input' option to use a GFM parser in the kramdown options documentation.