Feature/markdown parser

I saw that this project is using [rdiscount](https://github.com/davidfstr/rdiscount) as markdown parser. It's okay, however, there is an other good markdown parser I recommend to use. This markdown parser is [redcarpet](https://github.com/vmg/redcarpet).

This markdown is also recommended by github, you can read more [in this article](https://github.com/blog/832-rolling-out-the-redcarpet)!

When you first time use `redcarpet`, you probably need run `$ bundle install` before you generate your new post. 

If you have any problems when using this new markdown parser, please let me know.