Add support to creating posts in org-mode

The 'org' folder was added, which contains the subfolders 'posts' and
'drafts'. With a properly configured org-mode, any org-mode file in
these folders are exported to the corresponding folder (_posts or
_drafts) for Jekyll to process.

README.txt files were added in each interesting folder to clarify this.
Add a post describing the use of Org and Jekyll