Improve sorting algorithm, add unit testing, convert README to Markdown

Hi,

I hope this PR is ok for you since I didn't get a reply to #26.

The PR does the following:

* Adds unit tests via vimrunner - at the moment only the sorting is covered
* Improves the sorting function (I found a few bugs by writing the unit tests)
* It converts the README to Markdown since I think that code samples are easier to read when rendered with syntax highlighting by Github. If you want to see what it looks like: https://github.com/lenniboy/vim-scala/

The tests can also be automatically executed by TravisCI. If you want to activate it for _your_ repository, follow these steps: http://about.travis-ci.org/docs/user/getting-started/

All the best,
Leonard