Proper support for source code blocks.

Only language is supported right now

Pending future work is to support other options for the jekyll code
templates.

http://octopress.org/docs/plugins/codeblock/

{% codeblock [lang:language] [title] [url] [link text] [start:#]
[mark:#,#-#] [linenos:false] %}
code snippet
{% endcodeblock %}
Apply Yoshi's suggestions
Add variable org-jekyll-include-yaml-front-matter

Allow users to use rake files for creating .org blog posts that already
include the yaml-front-matter. The export will skip the addition of yaml
front matter when publishing if this variable is nil. The default is t,
which maintains backwards compatibility.
Merge pull request #13 from justin808/alternate-publishing-using-rake-for-yaml-front-matter

Allow alternate publishing using octopress-native source-block and rake YAML injection